$4 Dwarf isn't strictly better than Explorer as it can't gain Gold to hand, nor Bureaucrat as it lacks the attack. It would compare very well to those cards, but it isn't strictly better.
I'd like to see how well $4 Explorer without the Gold option does in a simulator.
Pretty badly. It's more or less the same as a terminal Silver that gains a Silver. Squire is a terminal Copper that gains a Silver but can also do 3 other things which justify its existence. Lucky Coin is a non-terminal Copper that gains a Silver which isn't on average that much weaker than Explorer and look how often you wanna play that card.
Or look how often you appreciate getting a Silver via opponent's Governor. Silver gaining is simply not the most brilliant thing out there in modern Dominion and, given Bureaucrat, arguably was never that good in the first place.
If Bureucrat's attack hits it is more or less the same as Explorer gaining a Silver to hand, if you wanna argue that, compared to handgaining, the topdecking is like
-1 Card and Bureaucrat's attack is also like
-1 Card.