I have 2 DP's in a heavily planted 6 UK Gallon tank with a few caves. I would only recommend 1 in a 4 gallon with maybe 2 otto's. This seems to be a safe bet as the puffers don’t seem to bother them and they will keep things clean.
They are very messy eaters. I use Frozen Blood worm daily and once a week i pop down to the local fish shop and ask for some pesky snails that you find in the plants section. They go MAD for these little buggers!!!
If you can use sand as your substrate as the puffers prefer is and it’s a little easier to clean. You can get this from most fish shops. (Don’t get Play, Builders or Sea sand!!!)
Cheers,
Neil