Is it possible to have a fuel tank that only holds three or four gallons of gas?

there is a bladder in the gen 2 tank that can be affected by age and temperature

 

That's right so there's something probably going on with the sending unit in the tank or there's something wrong with the gauge repair I believe that's possible so if you did it yourself you may want to go back and have a look possibly or take a look at the wiring for the fuel gauge and all that which is back under the seat in the back under a plate that's glued down It's a big gray plug you can pull up anyway it's a tricky situation because none of that stuff I don't think is replaceable You're stuck replacing the tank.

 

Just in case you find the issue is not with the gauges or senders, I had bladder issues on one Prius due to old age but the issue happened all of a sudden. It would literally shut the pump off after putting in a gallon or two into an empty tank at times.

My quote from that thread
"I get the symptom lately where it goes from 3 pips on the gas gauge to 1 pretty quickly and when I refill it takes about 4 gallons and reports back full or close to full."

When I posted here suggestions were to always refill when you have no less than 3 pips left. also never fill past the first shutoff on a normally working Prius. If you overfill it could get into some of the pipes behind the drivers side wheel well that work to trigger the shutoff
I think it may have helped but when I had fuel pump shutoff issues on a almost empty tank it sometimes helped to turn the pump nozzle upside down while filling

hopefully your tank bladder goes back to its functional operation and then never let it get below 3 pips and don’t top off