First of all nice lookig cham, he does look a tad over 4months nearer 5-7months. On the colour side male panthers dont achieve there full colour range till there around 1yr of age
I also have a BB ambanja and have been taking photos every month to follow the coloration changes.
If your cham is getting the colors in the same time period as mine, then he must be around 7 months. Mine just turned 7 months so no idea how he will lood from now on.