The 4' tank is probably the reason you've seen a drop in growth. I've seen several articles saying that a 6'x2'x2' should be the minimum tank size for adults, and even small loaches shouldn't be kept in anything less than a 4' tank, and I can back this up with my own experience. About 7 years ago (btw I've been keeping clowns for around 10 years) I bought 6 small clowns and put them into a 5'x2'x2' tank. In less than two years they were all getting close to 6". I broke the tank down and sold the loaches to a friend, who keeps them in a 4'x18"x18" tank. Today they are still only around 6".
My current group of large clowns are around 5-7", 4 of them were bought at jansens as "medium" about 14 months ago. They've all put on at least 5cm of growth in 12 months. My larger four haven't grown anywhere near as fast in the same time, as I think the tank size is slowing down their growth. They'll all be shifted to my 600L tank in about a months time, and my 14 smaller loaches into the 300L 4' until they're big enough to go in the 600L.
Its a well documented fact that their growth does slow down around 6", but perhaps yours wouldn't have grown so slowly if they had more room to swim.