Water Fountain Pump - GFCI

That's pretty normal for a ground fault in the early point of developing. It's only going to get worse.

You troubeshoot it in the normal way, unplug the suspect pump and see if the trips stop happening. If it does, there's your answer.

A sump pump is not necessarily designed for continuous usage. 4 hours is quite a bit. Many pumps are really designed to run for a few minutes and then wait a few minutes for the pit to fill and then pump some more, with continuous operation only in the worst of storms. If that's the case, it could be that overheating components might exacerbate any latent design or construction flaws in the pump, possibly to the degree that in laboratory testing/certification the problems might never occur.

Of course, if that's the case then repeated over-use may cause the pump to wear out quickly and/or fail in worse ways. Check the specs - there may (or may not) be a duty cycle listed. If there is, and you are greatly exceeding it, then I would see if you can put on a timer to solve the problem. For example, if it is rated at 100% usage for 1 hour and 50% usage indefinitely then run it 1 hour on, 1 hour off (or a shorter interval both on/off if you prefer). You might will likely find that you can push the stated limits a bit, but that extended continuous operation may simply not be practical with a typical sump pump.

Unfortunately, I just checked a few low-end sump pumps ($100 - $200 range) from different brands and none of them specified a duty cycle, max. run time, etc. But I think you will find that sump pumps are simply not designed for continuous operation.

That is a lot of power for a pond fountain and it is not intended service for a sump pump. There are a myriad of pond pumps , most submersible and some not . Look at internet pond supply like "Big Als" ( Canada). The most important factor for a pond pump is the strainer on the inlet, so you will need something. You want to stop junk from going into the pump but you do not want a filter to clean every few days. I have a 5 + year old pump , much smaller than yours but with 20 watts ( yours is likely 500+ watts) it pumps 500 gal/hour up 6 ft.I think it was about $ 50. The big savings in in electric power.