Ask dispatch to send batch ticket. Check tolerances.
Put down sand and compact.
Get the regular 3000 psi mix. 4k and above is overkill.
Fiber is good, steel is much better.
If using steel, make sure they use chairs or bricks to ensure steel is embedded in the middle of the slab. Do not allow them to tell you they can "raise it as they go". While in theory this method can be done, it's very difficult and few actually pull it off.
Do not let the finishers wet the mix beyond a 4 inch slump unless it has water reducer additives.
If pouring in temps over 80 or on windy days, request retarder and wet cure with plastic. If you don't you could get plastic shrinkage cracks in the first few days.
Hire a good finisher. Good luck.