I am looking at adding a sub panel, bur all Home Depot has for a GFI breaker to but in the main to run the sub panel. I need 4 15 amp breakers in the sub panel for christmas light and 2 smal pond pumps. Will this 50 amp breaker work?

Have you done a load determination? Most lights and pumps of this nature are well below 50 amps.

2 of the 3 amps will be running at 80% and the third 60-75% depending what I add this year. The forth will just have the 3 pond pumps on them, so very low as they all run on a 16 gauge extension cord without issue.

That is way more than enough.

Both these types of loads would be considered "continuous loads", as they would be expected to run for three hours or more.

Four 15A 120V breakers can supply a continuous load of 5760 watts.
A 50A 120/240V feeder can supply a continuous load of 9600 watts.

Thanks