It's pretty simple, really; just keep pushing for more. Alternatively, after you've maxed on the pullups, do negatives. But the only way to get a pullup is to train pullups.

I'm pretty sure you don't want to go to absolute muscle failure, though (looks around for Steve Low).

There is a technique called "Grease the groove" which is basically doing LOTS of an exercise throughout the day; say, every hour you do your max amount of pullups or thereabouts. I'm sure there is an actual technique to it, i.e. to figuring out how many to do at a time, and how often. Steve's "How to structure a workout routine" should be a helpful source for the specifics. However the point with GTG is that you are doing the desired exercise constantly throughout the day, with long periods of rest in between. That should help you a lot.

I hope to be able to do one pullup someday, so I feel your pain. Been working on it for a while.

Oh, PS: doing more pullups won't get you any closer to a muscle up. Theoretically, if you can do one pullup only, you can start training for a muscle-up because a pullup is, more or less, half a pullup. So since you can do 4 pullups, any articles you see about how to work up to a muscle up should be useful and you should start training for a muscle up if that's your goal, while continuing to increase the number of pullups you can do.

if you have strong door frames in your house you can grab a hold of them and do as many as you can each time you pass through a door. or if you don't have strong door frames, after you've done your pulls ups, hold it for as long as you can at the top of the bar, then come halfway down and do the same. that's what works for me. let me know how it works.

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