If some of you were around when Sanyo SCR NiCad was the big thing in racing, we charged the 1200mAH at 5amps then peaked them at 7 amps just before the race to warm them up so that they will have more punch. That would be charging at over 4C and then hitting them with 5.8C to peak them up. Still remember racers charging their SCR packs at 1.2A
If the Lipo packs are rated at 5C, then 4C can be used safely.
I don't like waiting and charging at a higher C also lowers the IR for better performance. I have a PL6/8 that can do 40A, and the Lipo packs are rated at 5C. No reason not to do it.