spa weight of 4,250 lbs
I have mine on a 4" thick reinforced concrete pad, no problem. If not reinforced, I'd go with 6". Basic cheap "Quickcrete" is 3,000PSI compressive force, plenty strong enough when you factor in the surface area of the tub's structure that will be contacting the slab.

Keep in mind though that concrete strength takes a long time to get to the peak strength levels. So don't place your tub and start filling it immediately.

Age  vs Strength percent


1 day    16% 
3 days   40%  
7 days   65%  
14 days  90%
28 days  99%


Oh wait, you said it was already existing... never mind.