As a side note, I love what you are planning with the system already in place.
With regards to what the 4 visible stats could do, I believe it would make more sense (feel free to use this or modify this as a base) with the stats doing the following:
STR^ - Stronger combo | could be a stronger combo multiplier (this stat could stack, based on rank, with the faster food timers and more money based on combo
END^ - Don't lose energy as fast and gain additional max energy per rank
SPD^ - Cooks food faster and faster combo rate
CHR^ - More money from working and a 3%/5%/10%/20% rent decrease