4ms is not out of bounds, but note when placed that close together can overdrive the radios and cause contention issues (if you look at the mesh link strength, it's likely VERY VERY high). Also if you're pinging with 5000byte frames, you're fragmenting it as well so you will see an increase in latency as well.
As far as any config, at that short of a distance, you just need to adjust power so as not to overdrive the radio, and that power will depend to some degree on the environment. I would start at or near max power, check the mesh link strenght (command depends on OS used), and if it's over 50 or so, drop the power. Make sure the APs are face to face when installed (we've had some users place them both face down) and they won't like well in that configuration.