Well, I think that's a good rate for "him". 4 inches from an inch isn't bad in 9 months, cichlids tend to be slow growers. If you raise fry, some of them will grow twice as fast as their brothers and sisters. It may be a superior gene.
JD are slow growers per say if equated to an Oscar, that grow fast for first 9 months or so. its about 2 to 3 years for a male JD to reach full size depending on diet, water quality and to some extent tank size.
The tanks dimension is 36"×13"×21", not sure what that comes out to in gallons. It's in their with just 2 bn plecos. I feed him a staple diet of nls and the occasional treat of bloodworms
As long as his color is good and he's active you're probably fine. I have 2. One grew pretty fast and is at 3-4 in. And the other seems to be a "runt" still like 2 in. So far as my research goes, some times it just happens.