Under a 400w, with those four plants you should get a decent yield. Anywhere from 3oz to 6oz would be my guess, granted they're all ladies.

For switching the light, just do it right before the lights come on, or right after, but be quick about it. Maybe have a green lightbulb in place so you can see(plants don't see green spectrums, so they'll think they're still in the dark).

I might recommend having one of your MH's running with the HPS if it's possible. I promise you're plants will love it.

P.S.-
MH spectrum is used for a plant to get better foliage and a better established root system(helps make squatter plants as well).

HPS spectrum focuses mainly on bud development.

This is why you use MH for veg, and HPS for flower. However, the combination helps immensly throughout. Maybe 2 MH-1 HPS fer veg, and 2 HPS-1 MH fer flower.

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Good luck,
Grow well
-Pool