Help with shop electrical

A buddy of mine (a member here that rarely posts) has a shop out on his land. It was existing when he purchased the property.
A while back, he began having problems with the breaker in the main panel outside (not attached to the shop) tripping the breaker on occasion. We replaced the breaker in the main panel and it ran fine for several months. Then the strangest thing, no power at all in the shop. House, well, and camper power are fine. There are 3 4ga (I think) leads running from the main panel into the shop, one is not used (taped off), so only 120v wired in the shop itself. Previous owners had one of the leads running directly to a bus, the other running backed through a breaker in the panel inside the shop. I tried bypassing the backfeed, as it really should not be needed. I have voltage after the breaker outside, but cannot get any voltage in the shop. The wires have continuity from the outside panel to the inside (disconnected from breaker and tested).
Im thoroughly confused. I can't think of a reason I cannot see 120 or 240 at the inside panel.
Any clues?

intermittent tripping could be a failure in the wire such as corrosion. The no longer working could be a final complete failure. There needs to be a breaker in the house panel feeding the garage.

Did you check the neutral when you checked the hots?

Overhead or direct burial?

We see this all the time in the country... moles/rodents chewing through wires under ground.

More information needed, is it overhead, underground direct bury, or underground conduit feed ?

Buried conduit i believe, but it could be direct burial, cant really tell. About 25 ft apart. He does have prairie dogs, but the nearest mound to the electrical is ~150yds.
I cant get any voltage across any combination of the hots/neutral.
I just bypassed the breaker in the building during testing.

Bet you the reason they only had 120v connected in the garage is one phase already failed so they disconnected it.

Ground heave. Freeze thaw cycle will kill it.

Only real choices are dig it up or trench all new lines

If in conduit or direct bury, there is something about the insulation on electrical wiring that mice just love and can't get enough of......
And as already said, pull in new wiring or trench it in.

If you are trenching, you might as well put in conduit to make it easier the next time.

you could find a guy with a fault locator. Likely the best solution is running new wire.