URD is not a listed wire type and may not be used. XHHW, RHHW, THWN are all listed wire types and may be used in an NEC installation. XLPE is not. Most of this 'mobile home feeder' is listed, but there is some out there that is not. The stuff the power companies use is not.
If its a listed wire type you can use it in conduit
someone more familiar with the applicable code section can tell you if the conduit has to be sized based on the overall diameter of the cable or if you can size based on the individual conductors since it doesnt have an outer jacket. im not sure.
practically speaking, if you are running 125ft through conduit underground you should consider getting individual conductors. i think pulling 125ft of 4 al conductors twisted together would not be fun. you may want to increase the size of the conduit a bit as well.