Well, you can "Cheat" by using extra wires to compensate for a smaller gauge then is recommended for a specific distance run. BUT, you have to double ALL wires, not just the "Hot" line. That means two wires each for the Hot, the Neutral, and the ground. That is a total of 6 wires.
In the case that you describe, it would be possible for you to cut the 250' cable in half, then have a double run of 125'.
IMPORTANT: 50 amp 4 wire service is actually two legs of 120 volts each that CANNOT be tied together because the potential BETWEEN THE LEGS is actually 240 volts, just like a household service.
You also might consider an Autoformer voltage regulator. Info here: