"This memory has been veried to operate at 800MHz at the low latencies of 5-5-5-18."
Beats me. Is it just a typo or something? But just incase if it isn't, would they still be compatible if 2 modules were 5-5-5-15, and 2 of them were 5-5-5-18, i mean couldn't I just set all the modules to be 5-5-5-15?
You already own modules with 5-5-5-18 timings. The modules at Novatech also are 5-5-5-18, not 5-5-5-15. You could try them at 5-5-5-15, but running 4 modules is more challenging than 2 modules, therefore you shouldn't run tighter timings.