I am setting up a small 4g aquarium. I have a large filter from my old tank that I want to reuse. Is there a disadvantage at putting 50g filter into a small tank?
If you put shrimp in the tank make sure that you have a protection against them being sucked into the filter.
If the filter is made for a 10 times larger aquarium it will consume a multiple of the necessary energy for the small tank. Check the power consuption. There're reliable filters for 20 l tanks with 2 W consumption. The investment of buying an (eventually second-hand) matching filter can compensate within one year already.
And like @Diether pointed out: the pump might be so strong that the whole aquarium will be a water strom.
Putting a 50g filter in a 4g aquarium is not good. There is a reason filters are rated by the gallon. It will be way to powerful for any fish you put in there. Small fish will be blown around, as if in a tornado. If you insist on using it, stuff the filter area with the extra filter pads you can get at fish stores to slow the flow way down.