I've had my 2018 SR5 for about 6 months. The most my fuel range has displayed was 380 after filling up. Did you purchase from a dealer? If so I would take it in to get checked out. Is your 4runner passed the 3yr/36k miles?
Hope you get this sorted without having to spend any money.
What is your exact problem? My 2011 Trail have been running out of gas even when it says it still has 1/4 tank or 50 miles. I've disconnected my battery for 30 minutes and reconnected. I haven't ran out since then because I haven't let my tank get that low again. Will update if it happens again.
I'll admit to being confused about your symptoms. Are you saying that the fuel gauge indicates full always AND that the digital range decreases down to a reasonable value like 100 after driving an expected distance??
If distance remaining decreases at a logical rate as you consume fuel then the sending unit must not be the problem since the distance remaining is a calculated value derived from the amount of fuel the sending is indicating.
For the original question: the sending unit is part of the fuel pump ASSEMBLY. But is not part of the fuel pump- the sending unit is a distinct replaceable part of the assembly.
The Techstream program has a module that allows simulation testing of the gauge. Also, you can get enough access to move the float to test for specified ohms without dropping the tank. There is an access panel under the rear seat.
If actually get to the low fuel light, mine triggers with 4 gallons left in the tank, takes 19 gallons to fill it. I have a 2015 and 2018 4runner and both are the same with the low fuel light.
Mine is a 2016 and has a 23 gallon tank. That is why I did not understand at a fill up why the range was only showing 370 miles. 370 / 23 is only 16 mpg. But after the ECU reset now shows 433 which is 18.82 MPG which is closer to what Ive been showing as my mpg.
I haven't let it go to the low fuel light yet but with the gauge working properly now i'm confident it will work like it is supposed too.