lipo & mtor ?
 

i have a hyper st pro w/ mmm , nue 1515 2.5d and i want to know if i can run a 4s5000 lipo in it as long as the c rating is a 30c or 35c ? or should i be running a 5s ?

What speeds are you trying to obtain..use the speed calculator to calculate speed, try different pinions, etc...great tool to answer many questions. Can plug in 4s, 5s and recalculate. The 30c batts should suffice

http://www.scriptasylum.com/rc_speed/index.html

I ran my 1515 2.5D on 4s for awhile. It doesn't have the amazing power that it has on 5 or 6s but it will get the job done. It does depend on your gearing though, I had mine in an E-Revo geared for about 35MPH and it was fine. I wouldn't gear it to high when running 4s cause it will draw more current with the lower voltage putting a higher demand on the batteries. I just put my 36-70-1600kv Medusa in my E-maxx and I'm running 4s lipo and 14s nimh. I only have it geared for about 30MPH, I wasn't looking for high speeds just the punch and torque of BL. Supprisingly I think it was faster on nimh than the lipo until about the last 2-3 minutes before the packs dumped. So long and short you can run it on 4s just don't gear it crazy!!!