Low Forward After L. Tatsu

Around 1:09 I noticed that he combos a non counter hit cr. mk after the l. tatsu. How is this possible given cr. mk is 5 frames and l. tatsu is +4 on hit?

It’s hitting on later active frames; the “+4” is for when you hit with the first active frame. A max range cr.hp xx LK Tatsu will hit on the 2nd or so active frame, letting you link into a cr.mk