A forward facing car seat is outgrown when one of three things happens; the child goes over the weight limit for the harness, the ears are level with the seat back, or the shoulders go over the top usable harness slot.
When forward facing, the harness should come from just at or above the shoulders. If it comes from below, its very dangerous and can cause spinal compression in a crash.
Read your car seat manual, if you have a 3-in-1 type seat (Alpha Omega, Eddie Bauer, safety 1st) the harness will only work to 40lbs, then it must be removed to be used as a booster seat with the adult belt. However these seats are typically outgrown in height long before weight, because they have a short shell and the very top harness notch is only good in booster mode. Plus these seats make terrible booster seats, they have consistently failed every crash test they've been in. Don't use them as booster seats!
At almost 4, 35lbs and 40" tall, she should absolutely still be in a 5pt harness. If she hasn't outgrown her car seat according to what I listed above, then leave her in it. She is absolutely safer in a harness because of her age and weight. If she's outgrown her current seat, you need to find her another seat. You can spend $50 on a booster, or you can spend $150 on a Graco Nautilus, which will harness to 65lbs, has a 19" top harness slot which fits most kids to age 7 in the harness, and then becomes a high back booster, and eventually a no back booster. With her only being 4, you have 4+more years of car seat use before she can sit without a car seat or booster, so invest now in a seat that will last her all those years while keeping her safest now in the harness. plus you can get the Nautilus in super cute pink patterns!
If you do insist on a booster seat iwth the adult belt only, please only purchase a high back booster seat. A no back offers no side impact protection, and children under age 6 or so are way too small to not have that added protection. My almost 8yr old is still in a high back booste rand will remain there... he is 60lbs and 53" tall. My 4.5yr old is sitll in a 5pt harness where he will remain for a couple more years.
The graco Nautilus is a great seat that will last her for years in the harness, and then becomes a good booster seat to 100lbs. For $150, you will get your moneys worth out of the seat, and keep her safest.