you can go into flower whenever you choose but personally I would like them grow a little taller. I usually go at least 6 weeks in veg. Just remember that they will still grow for a couple weeks when you switch to flower.
u have to take into account that u will lose about a ft with the pot. u need to be at-least 12 inc away from the canopy if using hid's, and u lose the height of the reflector. and she is going to dbl in size when u flip her if not more .
i would go ahead and flush them to prepare them for the flip. and then go ahead and flip them .
mine were almost the same size as yours when i flipped . i will show u a pic of what she looks like at 6 weeks in flower so u can get an idea . and i had her tied down for a week or so in flower.
u can remove some of the bigger fan leaves but if you going to flush then flip id say let her transition then u can clean her a little during the stretch .