Your fiddlers may get along but more likely will fight each other and anyone else.
The red claws are the ones to watch out for, however. They are very aggressive and will wage war on each other, the fiddlers, and plenty of other creatures, eating the losers.
There are exceptions, and maybe someone here will chime in with a tale of peace and love in their multi-crab tank. There's one way to find out which is true, if you dare to take a chance...
Otherwise, you may find yourself with four tanks for four crabs.