Sludge in tank.

Maybe there's just not enough pressure (from the weight of the oil) at 1/4 tank

Are there companies that will "polish" the fuel? I used to be responsible for backup power generators that stored lots of fuel but used very little unless there was a power problem. After a few years the generators would not run for very long when required because to fuel filters blocked with bad fuel - bacterial growth. The solution was to get the maintence company to "polish" the fuel every year. Basically pump fuel from one end of the tank, through a filter and then back to the tank. A few hours of this each year solved the problem.

Getting your current fuel "polished" could get you out of the hole. Anit-bacterial additives could be a long term solution if your fuel tank size and consumption is in reasonably balance. In the example quited above mine was not.

You can also try this.

But what @pecmsg says here ^^^^^^^^ is your best option. Your oil company should have a truck that can pump you out.