If you're using the '4psi Rule', set your cold start pressure & ride for an hour, then check again. An increase of
MORE than 4psi means your cold start pressure was
too LOW; an increase of
LESS than 4psi means your cold start pressure was
too HIGH; so an increase of 4psi is the 'ideal' target to aim for.

hyea:
But there are some things to remember - it's a 'Rule of Thumb', so it's not necessarily or specifically
exact, but for most, it
IS reliable, repeatable, and reasonably close for the majority of circumstances; a tire pressure gauge that's
consistent is better than one that's questionably or occasionally exactly
accurate; and adjusting your tire pressure is not the
only way of getting closer to achieving the ideal 4psi increase - briefly, you could consider adjusting your avg speed just a little, that can make a big change in tire temps & therefore in tire pressure; cornering, braking, and generaly just ryding harder or gentler can do the same; reducing or increasing the load on the tire can make a significant difference; and ambient temps, road surface temps, & road conditions (wet, dry, or anywhere in between) can make a difference! So you don't need to become a slave to checking tire pressures, but if you do check them regularly for a few weeks initially, just to work out what
YOUR ryding/driving does to your pressures, most will get pretty good at 'guesstimating' something pretty close to the pressure that'll give them that 'ideal 4psi increase' given what they are going to be doing today in today's conditions with today's load, and checking that you've got it fairly close to right every now & then is usually going to be enough. A TPMS can help make it easier to do that checking more often if you wish.
Just beware of getting too invested in keeping your tire pressures at exactly the 4psi above your cold start pressure - you're meant to be ryding safely & enjoyably, not sweating over getting your tire pressures exact for every circumstance &/or ryde!! Like Mike said above, there's been a lot of miles of testing & confirming, and many have done this often enough to confirm that for most running car tires as opposed to the OE Spec & differently constructed Kendas, there's just not that great a variation in the 'ideal pressure' from what's been recommended often, here & elsewhere. Still, even just checking your cold start pressures once a week and confirming that you're in the '4psi increase' ball-park once every half a dozen or even every dozen rydes is going to be much better than doing nothing, or just adhering to the (still better than nothing) tire placard 'one pressure fits all uses & circumstances' pressure, but if you do check for that 4psi increase about that regularly you'll probably see and feel the dividends in better ryde, handling, & tire wear, et al. Enjoy, but please, don't obsess!! :thumbup:
Ride More, Worry Less!