If you're not familiar with the Prius' hybrid stages, here's a good write-up on them:
http://techno-fandom.org/~hobbit/cars/five-stages.txt
Getting to S4 means you're ready to get max MPGs. Unfortunately, the ICE has to be warmed up
and idle while you're at a stop for ~5 seconds and turn off before going into S4 (don't ask me why). This is where knowing the coolant temp comes in handy. If you slow from a speed greater than 34MPH with the ICE warmed up, the ICE will usually turn off. Continue to a stop with the ICE off, and it won't go into S4. What I do is either blip the gas a little before coming to a complete stop to kick on the ICE, or when at a stop, hold the brake with my left foot and push the gas with my right to get the ICE on. Then it'll run for ~5 seconds and turn off, putting you in S4 (don't turn the ICE on if you won't be stopped long enough for it to turn off).
I also installed an EV mode button, not for trying to maximize my EV driving, but because it also acts as an ICE kill switch. The ICE doesn't even have to be fully warmed up for EV mode. I can P&G using the EV mode button and achieve pretty much the same effect as being in S4.