If you train your crew from lv4->6 with points and get trainer after that...
are you able to promote these lv6 crew -> lv7 with that one trainer?
I'm 99% certain that the 1 trainer you have will set your "max training level"
to 5 instead of 4...so you'd want to promote the guy to 5 normally, then use
crew points for 6+7.