Most buses do not have seatbelts (unless they are, like, Greyhound or other chartered buses, then some do, you have to call to find out.) Vests generally start much higher than your daughter's weight and one requires a tether, one requires it when used with lap belt only. There are add on vests for school buses, but they are not approved for public transportation.
However, really, buses are so safe that I don't see it as a huge need. Besides, think of what would happen to the one restrained child on the bus if it were to, say, roll-- she would be in danger from everyone around her becoming a projectile. It might be just as dangerous as being unrestrained (though it might not. But certainly very dangerous either way IF the bus were to roll. However, bus accidents just DO NOT HAPPEN that often.)
If you want her CONTAINED more than you really need her restrained-- you could use a carrier if she still fits in one, maybe even the backpack type that becomes a chair, or a stroller if they are allowed on the bus (often a small umbrella type would be allowed if there is room, and you can buckle her in to contain her that way.)
Think of this: EVERY time there is a bus accident, it is reported. When is the last time you read about one in that area? And, you can call if you want and ask if there has EVER been a bus accident on that route. The answer is probably "No" or "Not in 20 years"-- they are so infrequent.
I would feel completely safe taking my daughter on a ride on a bus for 4 hours at that age, assuming the company was reputable and the driver licenced properly, etc.