Theory: Take a $5 cantrip, remove +1 card +1 action, double the rest of the effect, and you have a $4 card.
Conversely, take a $4 card, halve the effect, add +1 card +1 action, and you have a $5 card.
Example: Remodel is a terminal that upgrades a card by $2. Halve that and make it a cantrip, and you have Upgrade. Take away Upgrade's cantrip and double its effect, and you have Remake!
I did this with a bunch of existing $4 and $5 cards. Some of the results are interesting and most of them surprisingly seem balanced.
$4
Cruddy Inn
+2 cards
+2 actions
Discard 4 cards. When you buy this, shuffle any number of action cards from your discard into your deck.
An extremely punishing village-warehouse. Play this and you'll end up with a 2 card hand. Worth it only really rarely.
$4
Cruddy Market
+$2
+2 buy
Like Woodcutter, but with far too much +buy. Maybe crazy Highway strategies will want this.
$4
Cruddy Treasury
+$2
When you discard this from play, if you didn’t buy a Victory card this turn, you may put this on top of your deck.
I think this very card has been proposed, at this price, before, though without the no-Victory condition. I think it would be a lot of fun, and very strong if there are no other good early terminals, or an abundance of +actions.
$4
Cruddy Highway
While this is in play, cards cost $2 less, but not less than $0.
Too similar to Bridge really. Stronger in theory but with less self-comboing. Most of the time I'd prefer Bridge.
$4
Cruddy Cartographer
Look at the top 8 cards of your deck. Discard any number of them. Put the rest back on top in any order.
What Scout should have been. Not strictly better (no +action), but will make your next turn near-perfect on average.
$4
Cruddy Bazaar
+$2
+2 actions
Festival without the +buy. Or a silver-village. Really good - I'd be more likely to use this than Festival. You could quickly set up an erratic draw engine with Oracle or Smithy without needing to hit $5 or buy money.
Laboratory, Hunting Party, and Stables (all lab variants) become shockingly weak when converted to $4 terminals. Laboratory becomes a vanilla "+2 cards"! It goes to show how much stronger +card is when it's in a cantrip.
City just becomes weird when converted. The $4 version of two-pile City is better than the $5 version! Although the $4 no-pile City is just "+2 actions"
$5
Super Thief
+1 card
+1 action
Each other player reveals the top card of his deck. If they revealed a treasure card, they trash it. You may gain any or all of these trashed cards. They discard the other revealed cards.
Way better than Thief. It'll miss a LOT but you don't sacrifice your hand and action point just to put pressure on their economy.
$5
Super Scout
+1 card
+1 action
Reveal the top 3 cards of your deck. Put the revealed Victory cards into your hand. Put the other cards on top of your deck in any order.
Much better than Scout. This is a Lab variant. Better than Lab if more than 1/3 of your deck is victory cards. I didn't quite halve the effect as I normally do because Scout already has a +action, all he needed for a cantrip was +card.
$5
Super Navigator
+1 card
+1 action
+$1
Look at the top 2 cards of your deck. Either discard both of them, or put them back on top of your deck in any order.
One of many $5 peddler-with-a-bonus cards, and one of the best. Zoom through your deck AND filter it while making money and losing no handsize or actions, how can you go wrong? This is better than Cartographer imo.
$5
Super Silk Road
+1 card
+1 action
Worth 1 VP for every 8 victory cards in your deck, rounded down.
A Great Hall variant. Only worth it if you can get it to 2 VP, and that's hard. 8 of these, 4 provinces, 3 estates and you're still short!
$5
Super Trader
+1 card
+1 action
Trash a card from your hand. Gain a number of Silvers equal to half its cost in coins, rounded down.
—
When you would gain a card, you may reveal this from your hand. If you do, instead, gain a silver.
Similar in purpose to Upgrade. The drawback is you can't get any fun $3 instead of silver for your estate. The upside is the useful reaction effect. Then again, lategame Upgrade is probably better. Trashing a witch will give you 1 gold with Upgrade, 2 silver with Super Trader. Overall a wash with Upgrade. Depends how well you can use silver really.
$5
Super Baron
+1 card
+1 action
You may discard an Estate card. If you do, +$2 and +1 Buy. Otherwise, gain an Estate card.
I made the +buy conditional to "halve" it. This guy is exactly a Grand Market if you discard an Estate. Otherwise he's horrible because you have to give up the cantrip effect if you don't want to gain an Estate.
$5
Super Gardens
+1 card
+1 action
Worth 1 VP for every 20 cards in your deck, rounded down.
Another Great Hall variant. Again very problematic to get to 2VP - you need 40 cards in your deck! Even then, it competes with Duchy. When exactly would you buy this?
$5
Super Horse Traders
+1 card
+1 action
+1 buy
+$1
Discard 1 card
When another player plays an Attack card, you may set this aside from your hand. If you do, then at the start of your next turn, +1 Card and return this to your hand.
Market with an extra advantage and drawback. Not worth it unless there are a LOT of attacks going around. Even if the reaction goes off, you're basically getting a 1-card filter total compared to market and that's it.
Combos with Tunnel and Library and other sicko cards that LIKE discarding effects.
$5
Super Envoy
+1 card
+1 action
Reveal the top 3 cards of your deck. The player to your left chooses one for you to discard. Draw the rest.
Seems insanely strong. +3 cards +1 action in total! We saw when trying to convert Lab to $4 how strong +card is with a cantrip. Now we see it the other way.
$5
Super Salvager
+1 card
+1 action
+1 Buy
Trash a card from your hand. +$ equal to half its cost, rounded down.
If you trash an estate this is a Market. Plus it gets rid of an estate! The problem comes when you start running out of junk in your deck. With Upgrade and Salvager this is (often) no problem because trashing even good cards continually improves your deck with these. But Super Salvager reduces the value of whatever it touches. And like Upgrade it has the conundrum that you have to play it if you want to see your next card, and maybe hope that it's junk. IMO too risky to buy unless it's the only trasher.
$5
Super Monument
+1 card
+1 action
+1 VP
A Great Hall that's worth 1VP for every cycle of your deck. Much stronger than it looks IMO. Crazy in draw-your-whole-deck decks. It'll be worth 1VP per turn in the game!! Alternatively..
$5
Super Menumont
+1 card
+1 action
+$2
Another crazy good card. Compared to Market you sacrifice +buy for +$. Who wouldn't do that?? This is the card Donald originally intended Grand Market to be, and he priced it at $6 (correction: $7!) without the no-copper condition. So this is clearly strong for $5. It seems that no matter how you halve Monument, it looks great as a cantrip. No wonder it's so strong in its terminal form.
$5
Super Moneylender
+1 card,
+1 action
Trash a copper from your hand. If you do, +$2.
Really strong early. Seems too strong for +$2 but too weak for +$1.
$5
Super Militia
+1 card
+1 action
+$1
Each other player discards down to 4 cards in his hand.
Yet another peddler-with-a-bonus. Normally a kind of weak bonus, but hey, so are most of them. Almost everyone is going to have a curse or VP card they don't mind tossing. But this would be great with Council Room and Governor, much moreso than normal Militia because it doesn't shut down your +action/+draw engine.