SVC: Street Fighter Characters on 4 buttons

Hi, I was wondering if I could ask a question to those of you lucky enough to play this game already, how do the street fighter characters play with only a four button layout? Does it work like CVS1 where holding back and hitting fierce will do a medium attack? Are there no medium attacks at all? Is it a whole new system all together? I really want to know, because many street fighter strategies revolve around the medium attacks (Vega is who I'm mostly concerned with). Thanks in advance!