Are all the trailheads that are accessed from highway 173 ok to drive without a 4 wheel drive?

Yes. The highway turns to dirt at the end but that is where you park and hike from.

4 wheel drive only matters if you don't want to pay the $10 per person fee or not. If you have 4 wheel drive keep going straight before you turn on the property owners land for parking, it will take you around his property without having to pay the fee. It's a shorter hike to the hot springs as well, but it's all up hill on the way back.