PLEASE HELP almost 4 and 31 lbs - Is he too small to be in a low-rise booster seat?

Safety wise he should definitely be in a high rise.  Technically, I believe most boosters are recommended for 40 pounds and up.

Not sure if it's every state, but pretty sure it is. 4 years and 40 pounds to be in any kind of booster (meaning just the seat giving them a boost and using the car's seat belt over them). You can google your state and car seat laws or something like that and it comes up.

Look at the sticker on the booster, each seat if different.

 

 Thank you all so much!

Safety wise he should still be harnessed. Under 40 lbs they are at risk for submarining out from under the lap belt.
My son will be 5 in June and is 38 lbs. He is harnessed in a britax frontier85. He will outgrow that in height within the next 2 years and will go to a booster at that point

Google your state laws but at 31 lbs I'd keep him in a 5 point harness seat.

http://www.safekids.org/in-your-area/safety-laws/child-passenger-safety-law-southcarolina.html

I looked at your profile and saw you live in South Carolina. Googled for you. Under 40 pounds needs a harness seat.

Federal law states a child must be 4 AND 40 lbs to be in any type of booster seat. Until they reach BOTH requirements, they must be in a 5 point harness.

 Wow! You guys are all so helpful! Again, Thank you so very much! In a five-point harness he will remain!:)

Quoting ELFs_Mommy:

Federal law states a child must be 4 AND 40 lbs to be in any type of booster seat. Until they reach BOTH requirements, they must be in a 5 point harness.

LOL Actually there is no Federal Law in reference to booster use.  Each state governs themselves....

OP don't worry about what the law is is just not safe.  Keep your child harnessed as long as possible.  every step "up" in seat is a step down in safety. Depending on your budget and your child's torso height there are seats available to keep harnessing.