SF4's footsies are kinda diminished from other fighting games for a variety of reasons, but the idea is still the same. Other than the characters that have very fast walk speeds or dive kicks (where you can mix up footsies with quick off-the-ground dive kicks), I don't particularly think they're as useful as they are in other games, but, of course, they're still a viable mind game.
The easiest thing to try in SF4 is to take advantage of people's tendency to mash jab or throw tech (crouching throw tech always results in a crouching LK) during block strings by spacing your attacks slightly as you get pushed just outside their attack range. For instance, you can cross-up MK with Ryu, and on block, do something like cr.LK, cr.LP, cr.LP, pause for a split second to allow your opponents jab to come out and punish the whiff with cr.MK, wherein you can then score some damage on them where previously you were getting pushed out of range in a pure block string. If you're too close (or you try this on someone stupid like Balrog with his insane jab speed and priority), you're gonna get stuffed by whatever move they were trying to counter with, so the idea only works when you know your opponent's attack range in and out.
This also works to counter people who love to mash dragon punches during block strings (which is like, my most favorite thing ever).
A bit more difficult is to remain in attack range and take advantage of positive frames on your normals to space your attacks so that you allow time for the opponent's attack to come out, but not enter active frames before your counterattack does. You can punish crouching throw tech misfires very easily this way too. Same scenario: cross-up MK, cr.LK, cr.LP, pause for a split second, cr.LP, cr.MK. Ideally, what will happen is that between your first cr.LP and second cr.LP, your opponent will try to jab or tech a throw attempt and your slight hesitation allowed just enough time for the attack to start, but not hit -- you then counter their action and can then hit verify the cr.LP, cr.MK, and do something like EX fireball -> FADC -> Ultra1 for good damage.
This one is beaten by reversal dragon punches and the like, however, and you don't ever want to try this against a character with a command grab unless you've first trained them to expect a jump after the hesitation. Even then, probably not worth the risk.
If you're playing a character with a fast walk, it's much easier to do the first element in Iconoclast's link: stepping in and out of attack range to bait pokes and punish. I don't think this is very effective for most characters. That detail seems to make it even more effective for the characters it does work for, though.
