Well yes, this would be ok. Facebook allows files up to 2048 pixels, and you only need 1500 pixels for a 6x4 print.
In fact, if you uploaded them at 2048 pixels, people could even print 5x7s if they wanted.
Is the organisation aware that Facebook images are somewhat reduced quality? I guess it won't matter, eh?
Do they want you to put a logo on the photos?