Can you check the size of your fish?
The fish does not look like it is four and a half inches long. My guess is the fish is about 2 inches and is a young male.
Melanotaenia praecox (neon blue rainbows) only grow to about 2 inches maximum and the boesemani is a similar size the male M. praecox.
Male rainbowfish produce a dorsal blaze, which is the gold, white or pink stripe that flashes on and off when they breed. This stripe is located on top of the head and runs from the start of the first dorsal fin and goes down to the nose. Female rainbowfish do not produce a dorsal blaze.