Tekken 4 didn't take on Soul Calibur's movement system. Soul Calibur uses 8-way run. Tekken 4 just added a side walk,
not an 8-way run system.
The problem with Tekken 4's side walk system was that you could buffer it after any move. Say for instance you hit your
opponent with a 2 punch string (1,2) and they block it. While attacking your opponent you can hold up or down so you
side walk as soon as you recover making it hard for your opponent to punish you, even if they are at the
advantage.
The biggest problem with Tekken 4's movement was that they got rid of korean backdashing which is a way to back dash
over and over without having to recover (b~b~db, b~db, b~db etc). Because of this, Nina was the only character who could
control spacing with hayashida (qcb~u, qcb~u etc). This was very stupid.
Tekken 5 forces you to side step before side walking so you can no longer buffer into side walk. More importantly, they
brought back korean backdashing.
Also, to crouch in Tekken 4 you hold downback instead of just down.
I play Tekken competitively (I live in Houston so I get to play against Crow, the current Tekken 5 champion) so if you
have any questions I'll be more than happy to answer them as they boost my ego and make me feel like a man.