The plants are almost 4 weeks into veg, and the cotyledons on some of them have gone yellow. They've not had any nutes at all so far, so I'm thinking maybe now is the time to start? I've got either 18-4-7 or 5-5-5, I was thinking the 5-5-5 at half or quarter strength, does that sound appropriate?
Well im no professional or anything but i use a 10-15-10 for my plants and had to give them extra nitrogen (blood meal) so if i were you i would start out with the 18-4-7 and use it at 1/4 strength buts thats just my unprofessional and non-experianced answer.
when those cotyledons turn yellow is an indicator of when to start ferting. like shimen said, whatever you choose to use for nutes start off at 1/4 (or even weaker) strength and see how they react. slowly work your way up to full strength. high N for vegging and high P&K for blooming. good luck.