I am having lower back pain and have had it for four days now. I keep going to the bathroom to pee. And I feel like I am not emptying my bladder all the way. And I have to go again in five or ten minutes.

Could be kidneys but I would see a doctor to be sure and get the right treatments and meds for whatever the issue is.

I agree about kidneys. If an infection backs up into them, it can be serious.

BUT it can be many, many things.

In my case, I've had lower back pain and a constant urge to pee for many years. It's nothing more than some scarred sacral nerves hitting my bladder, and other nerves pinched as I get shorter in old age.
The feeling that it's not all out seems to be common among female friends of mine (we are age 70+). I sit there for another 2 minutes, and get more out, sometimes twice.
HOWEVER, that can also be a sign of a kidney problem.
In my case, my blood tests were all within normal range, so I'm not going to pursue it, even though I have another sign of aging kidneys, the persistent cough from taking certain blood pressure medications (the kidneys can't deal with byproducts of the drugs so they send them to the lungs).