they actualy grow really slow. slower than any other fish *** had. i had one that was 2 inches like yours looks and it took almost 2 years to double in size. i feed them 2 times a day though the more experiance i got i realized if u feed them alot and i mean like 5 times a day just a small amount they grow alot faster so give that a try for a while and see if he shoots up. give him just a few pellets each time u feed him u dont want to over feed him or he wont eat every time u try to feed him then ur tank will small cuz the food that wasnt eaten haha oh, also if its a female then it will grow suuuuuper slow mine grew so slow and stayed pretty small
Growth rate is primarily dependent on three things: clean water, food, and temperature. Knowing the proper temperature range for the fish influences the effectiveness of the first two. Keeping them on the warm side of their range, more food and more water changes will produce faster growth - and a shorter lifespan. Keeping them at a mean temperature, feeding a moderate amount, and doing your weekly water changes will produce normal growth and life expectancy. So basically, you have to decide what trade off you are willing to make.
My experience with Herichthys is that they are all fairly fast growing fish, and taking two years to reach 4 inches suggests too little food, too cool a tank, or not enough water changes.