4 Plants in different stages in one tent

They are all auto flowers but i heard they get more colas and crystals if i use the right lights

Ive tried it and you can, buuut theyll be very small and will affect your yield massively lol. I would recommend to wait for them to get bigger in veg and keep low stress training the oldest 2 as they wont flower untill you switch the light to 12/12.

No, the seedlings will also go into flower. Unless you have another space to put them in to carry on with the veg cycle for the small plants your only other option is to veg all of them until the small once are at a good size. You could put up a scrog net to keep the big ones down and not overshadow the smaller plants until the small ones catch up.