Ok, in a 4 month old tank, unless there has recently been a major disturbance (i.e., moved the tank, fallen power head blew the sand bed everywhere, etc.), I would disregard the positive nitrite reading. It is almost certainly a false positive as nitrite tends to get scavenged pretty quickly if there are sufficient nitrifiers present, and 4 months is plenty of time for nitrification to get up to speed.
Nitrate tends to decrease over the course of months to about the first year or so in most reef tanks, as algal biomass increase (especially coralline and turf algae) and as food webs become more complex and more stable (mostly little critters--various worms, crustaceans, sponges, etc.--and microbes). However, it also depends heavily on the filtration system and fish biomass (or rather, the amount of fish waste). For a 4 month old tank, nitrate at 5 ppm doesn't seem bad at all to me. I'd continue to apply good husbandry (what that entails takes at least a book to describe) and the tank should continue to stabilize and improve in conditon.
cj