The size is pretty limiting, though you do have a few options. You could do Endlers, which are similar to guppies but a little smaller (and prettier imo). I hear that Bettas can do well in a 5 gallon, but you would have to look into that more. Some sources say that you can house 1 or maybe 2 clown killifish in something that small, but there are conflicting opinions. You might want to look into small killifish, as they seem like they would do well in a walstad tank. Maybe some of the nano tetras or rasboras. Maybe dwarf cory cats, but I don't know much about corys.
On many smaller aquarium store websites, you can find a section called "nano fish". This could be a great resource, but just make sure that the fish will thrive in a 4.5g. You can also search thus sort of thing up on youtube.
Hope this helps.