questions about lighting hps and MH

thinking of getting a 400w hps light can i put a 400w MH bulb in it?
or should i get a 400 MH ballast and stick a 400w hps in it
not sure how the ballast works, hoping someone can break it down for me

also im growing autos so is it worth having mh on them for 3-4weeks then hps for the rest or will they be fine with just hps for the whole grow period,

will be growing lowryder original and auto blueberry

i will have 5 plants under one light

get a digital ballast and it can do both, I recommend quantum, you can use just a hid for the whole grow, although a mh first works a little better.

cant really get a digital ballast as they start at like 200$ here in australia, but i found mh conversion globes that run in hps ballasts so problem sorted