I think based on the info at hand. You say you feed her rats every 7-10 days. Assuming they are appropriate sized rats, at the age she is (or you claim her to be) she should be bigger. I just got a 2 year old girl and she's already 2600g. Considering that blood females need to be about 5000g to make babies, at 4 years old they should be around that mark.
Males are much smaller than females and the age and size of your snake seem to be right on track for a male. Of course this is all speculation, but you could get your snake sexed to make sure.
Definitely not skinny. How big are the rats that you're currently feeding her? If she got off to a slow start in life (i.e. lots of small meals) and she's not eating especially large meals now, it's going to take her some time to catch up/bulk up in terms of overall mass. It's also possible that she's just a smaller animal - some lines/individuals fall on the smaller side of "average" in the grand scheme of things.
I would try to get it sexed so you can know for sure. It may be a male, could just be small, or started out slow. I would recommend getting its current weight so you can make sure it is getting the right size meal.
A probing kit is not expensive and I found it very easy to use. I bought one to verify three bloods I bought were the sex I was told. I bought them to some day breed so I wanted to make sure I didn't waste the next few years getting them to size just to find out they were all boys or girls.
Also Amazon has cheap scales. Just remember to look at their range. Make sure it can weigh your snake now and continue to in the future. My blood outgrew the one I have so I need to get another.
KMG
0.1 BP 1.1 Blood Python 1.0 Brazilian Rainbow Boa 1.0 Aru Green Tree Python
0.1 Emerald Tree Boa 0.1 Dumeril Boa 0.1 Carpet Python 0.1 Central American Boa
0.1 Brooks Kingsnake 0.1 Speckled Kingsnake 1.0 Western Hognose
0.1 Blonde Madagascar Hognose 1.0 Columbian Boa
He/She is beautiful either way. Not rotund like they tend to get but by no means skinny either. +1 on rat size Q. I feed mine retired breeder size usually.