4x5 is awfully way too small for 13 birds IMO....unless maybe you have a large, weather and predator proof run and live in a milder climate. There's a great article on Space by Ridgerunner link in my signature.
The '1' of roost length per bird' is a rule of thumb(def. a broadly accurate guide or principle) that does not always take into consideration variables that might exist.
If birds have to fly up to roosts they need more space to accommodate wing spread.
If you have a poop board 12" or less under roosts, you can get away with less length as they can navigate on the board and step up onto roosts as well as roost on the board itself(depending on configuration). That is why my 11' of roosts shown above can accommodate 25 birds...tho 11 of those are 15 weeks old and space is getting tight as they grow, 5-6 sleep on the 'cleaned every day' poop board. My flock population will be down to about 16 by the time winter hits.
Here they are when chicks were just 6-8 weeks old.