It is possible that your dragon is 4 months old, you really never will know for sure though. It's very difficult (I'd go so far as to say impossible) to accurately gauge age by size.
Either way, whether your dragon is 1 month, 3 months, or 4 months old doesn't matter too much
As for the eating question,
You should be offering a fresh salad every single day. I like to add a salad first thing in the morning for them to munch through on all day.
For a baby, you should be offering live insects 2-3 times per day, and AS MUCH live insects (such as crickets/dubia/PW's) as your dragon can eat in a 10-15 minute time span. This can equal over 50 appropriately sized crickets per day!
Baby dragons especially need lots of protein in the diet to keep them on a good growing pace into adulthood.
And as for weight, ideally I'd like to see a dragon a bit heavier at 5 and a half inches, but if you are feeding him like how I mentioned already, then it's ok.
-Brandon
