I agree the U blade or tree grubber attachment is not your answer to dig. I second the stump bucket option.
I have a heavy duty stump bucket I use on my MX5800 and track loader, for your tractor I would recommend the following:
The Titan stump bucket for Skid Steer Quick Attach is perfect for uprooting tree stumps, lifting concrete slabs, and for digging footing around a house.
www.palletforks.com
It's weight is only 151 lbs.
Or you could also get away with the heavier model which is 470lbs. This is the model I have and it is good for digging on my MX5800, it is possible to dig a trench but is a little messy. The tractor loader does not have enough lift strength to use it as a heavy duty stump bucket but it will work great with your L4701 for helping you dig. Once I put this on my track loader it is possible to quickly remove many stumps. Remember these attachments are designed primarily for skid steers / track loaders.
I believe your loader is a LA765, it has a lift capacity at 500mm forward of the bucket pivot pin of 1263 lbs. Be careful how you use it so you do not damage the loader frame.
It comes down to how far down do you want to dig, what type of ground you are digging and your methodology of using the attachment. On a tractor loader I found the best method was a steep entry angle, roll it back and drive forward at the same time and scoop out dirt.
Good luck with your selection.