Wire and Conduit for line between house and outbuilding
If you run conduit you would use individual wires. You could run the entire distance in conduit if you choose.

A 50 amp circuit only requires a #10 ground so you would have 3-#6 and 1-#10 copper wires for this circuit. These would fit in a 3/4" PVC schedule 40 conduit according to the Code.

With a light load (4 KW is less than half of this circuit) voltage drop should not be an issue but if you add a large motor load like a compressor you may get a slight flicker in your lights when the compressor starts. The longer the distance on the circuit the more this becomes an issue. Since you are currently oversized it should not be a problem unless you plan to put a larger load on the end of the line in the future.

Good luck and be careful!