So when you're talking four bet-bluffs, are you talking pre-flop or post?
At micro stakes, I've found so far:
Pre-flop: most who 3-bet will hate to fold their hands, as a result, any hand they 3-bet is a hand they will go all in with, and because of this, they can have a range of pretty much anything. I've seen people go all in with tiny pocket pairs or suited connectors, when they are a coinflip at best against the over-cards.
This means 4-bet bluffs have tiny equity at microstakes level, the only way they will work is if you're the first one to ship, and even then be prepared to be called by a lot of 3-betters, and hope to hit your cards.
Post-flop:
Again not a good idea to 4-bet bluff, you will often get shoved on, especially HU
Players at micro levels tend mostly to play their cards post-flop. If they hit anything slightly above marginal, you wont get rid of them unless you are repping a big hand both pre and post-flop. They are not concerned with your range mostly, just what they have. As a result, i'd try some 3-bet bluffs post-flop on a c-bet, but if you encounter resistence and you don't have much, aim for a cheap showdown or fold