When you say you know the basics, do you know ALL of your characters normal moves by heart (including neutral jump attacks vs. diagonal jump attacks, etc.), and which ones apply in what situations (what your anti-air is, what moves have high priority, what links into what, etc.)? Do you know all their specials and what each special can be canceled into from? These are all EXTREMELY important foundations you need to know to succeed, because they are for one thing a huge part of your game on both offense and defense, and are the fundamentals to all combos.
Moving on from that, playing ranked matches online really won’t help you, as one match against a different opponent with a different playstyle one after another won’t teach you anything. What you need is a sparring partner, which can be found on these forums (or better yet get one of your friends who is starting to really get into the game to come spar with you, offline > online), who can play with you over and over. This will teach you to learn to read your opponents playstyle and how to adapt to it, which transfers over from opponent to opponent far more easily than you would think.
Also, something I should have mentioned at the start is that you MUST know the range of all of your attacks, that includes normals, specials, ultras and super. This is important for both combo building as well as playing footsies. Learn the pokes of your character to properly play footsies, and check the stickies in this forum as well as the topic in the “Fighting Games” forum on this site called “Footsies guide…” (you can just search for it). These will teach you the basics of zoning and footsies, which are EXTREMELY important in SF4.
And finally, choose ONE character and stick with it until you’re comfortable. You will not improve in the slightest alternating between characters, you have to completely learn a single character before you can get anywhere in this game.
I know these rules sound strict but it’s by far the easiest way to improve.