i view sf4 almost as sf2. its very footsie and zoning intensive. because there is no parrying, the matches are typically slower paced with both players playing a defensive game while opening up when given the opportunity.
the focus attack and parry are like far cousins. the focus attack can be viewed as a remix of the parry. they both do something similar in concept but are used for vastly different reasons. u can focus normals, projectiles in order to gain position or retreat. or u can fadc specials in order to continue combos, or trap opponents (picture a classic ryu gameplay= keep away hadoken, shoryuken aa, hadoken fadc shoryuken aa trap).
the focus attack absorbs one hit, however certain special moves have armor breaking properties and can shatter your focus attack. if your opponent combos a 2 hit combo, it will shatter your focus. the focus attack is an attack, just with a system around it. the focus attack is built around 3 levels. [level 1] you can press it and have the attack come out normally. [level 2] u can hold it momentarily in order to absorb an attack with and crumples opponents if it counts as a counter hit. [level 3] u hold it for max length and it crumples opponents no matter what.
focus attacking should not be abused, however using it efficiently will level up your play. when faced against an opponent focus attacking, it IS possible to focus THEIR focus attack and counter hit. many shenanigans are used with the focus attack like charging a focus attack before an opponent stands up and back dashing baiting a reversal. or 2 opponents charging a focus attack at the same time playing a game of chicken to see who will quit first. or focusing a jump in attack and back dashing (not really a shenanigan).
Leveling up your footsie game will improve your game and help u incorporate focus attacks as a tool to use along side your pokes. its very common to see a very well placed focus attack absorb a hit, and counter-hit crumble an opponent leaving them open to a super/ultra/ or some other kind of punish. id highly recommend reading this link as it will improve your game tremendously.