Donald X. has said many times that testing has shown cards that are strictly better than Silver cannot cost $4.
Regarding an early version of Caravan, Donald X says "Caravan was where I learned that Silver with a bonus couldn’t cost $4." (source)
I'm trying to find where he talks more about why. The Secret Histories are long though.
Part of the reason really is that $3 and $4 are not all that different; the real break point in pricing is $5. Certainly there is a difference anyway, and we see that with many cards (e.g. Village and various $4 Village-with-bonus), but a key difference is that Silver is in every game.
Edit:
"Caravan:" Only it's Merchant Ship. This replaced "+$2 +2 Actions" for $4. I can't do "+1 Action +$2 something something" for less than $5 (without a penalty), because you just automatically buy it over Silver in most games.
(source)
Sure, but I think soulnet makes a good point. This variant is not any better than Silver when purchased. The only benefit is when gained during the action phase. Now, first of all, there are plenty of boards in which this never happens. In that case, its extra bonus will be worthless, as Fortress and Rats sometimes are.
What are the ways of gaining a $4 treasure, but not a $5? Workshop/Ironworks/Armory. Remodeling an Estate.
There are some others. You could Expand a Shelter, but man, that doesn't happen often, and by the time you have an Expand this is probably fine. Trashing Catacombs, but this doesn't feel so broken to me either. Then there are cards like Develop, Upgrade, etc which deal with exact costs, so there are plenty of times when you could gain a Silver but not this variant, or a $5 variant but not the $4 variant.