I believe some more explanation of the terms would help you understand this one way better
When comboing you can either “link” or “cancel.” For this explanation when I write something like…c. LP ~ c. LP I mean that you need to link the moves together, but if I say something such as c. MP xx HP Headbutt that means you need to cancel the move
A “link” would be throwing out your attack and waiting until the animation finishes or recovers and then hitting your opponent with another attack while they’re still in “hit stun.” Combos such as c. LP ~ c. MP with Balrog is strictly a link, you cannot mash out the c. MP or it won’t come out. Links require timing.
A “cancel” on the other hand does not require much timing. “Cancels” are something such as c. MP xx HP Headbutt, where as soon as the c. MP hits you can input HP Headbutt and it will hit. This is because the special move effectively cancels the animation of the c. MP, starting up your special while the opponent is still in hit stun, allowing the moves to combo. You’re not waiting for the c. MP animation to end, it just goes right into HP Headbutt.
The way that combos work in SF4 is that while you can cancel your c. LP’s into each other (aka spam, not time) it doesn’t allow you to cancel normals into a special unless you linked the hit that you are trying to cancel into a special. So that’s why you can do something like c. LP xx c. LP ~ c. LK xx Headbutt, because even though you canceled into the second c. LP you got that link in at the very end and are allowed that cancel into special
Hopefully this helps more than it confuses you, if it does confuse you it’s because I just explain things badly