A 5pt harness is always safer, as it restrains the body in 5 points, instead of just 3 in an adult belt. This means less stress on the body as the crash force is distributed across the strongest parts of the body instead of just 3. If you look up booster seat crash tests and car seat crash tests on youtube you will find alot of good crash tests to see both sides of how a child is held into a car seat vs a booster.
That being said, I don't think any 3, 4, even most 5yr olds are ready for a booster seat. With the vast selection of seats out now that harness to 65 or even 80lbs, there is no reason for a child under age 5 to be in a booster seat.
There are a couple options for your daughter at her age and size.
The Graco Nautilus is a great seat. It is $150 at Wal Mart or Target, and it will harness from 22-65lbs forward facing only, an fits most kids in the harness until around age 7, then it becomes a high back booster, and if necessary down the road a no back booster to 100lbs. For the price you will get a seat you daughter can use until she is big enough for the adult belt (8yrs old, 80lbs, 4'9" tall). The harness portion of the car seat is good for 6yrs, and the booster portion is good for 9yrs. This seat will last through two kids.
Britax Regent will harness to 80lbs. We have htis seat, my 7yr old has no problem fitting in it. Its a beast, but its so comfy! Like a recliner for the car! Its top harness slots are 19" and most kids can easily ride in this seat until age 8+. You can find it on the cheap now as the model has been discontinued, so you ca nfind it for $150. Its a great seat, I highly recommend it.
Britax Frontier will harness to 80lbs. Its top slot is a bit lower tha nthe other two I mentioned, but its a great seat tha tmakes a fabulous booster down the road. It run around $280, so its a haul but its a great seat.
Ther are a few convertible seats too that will harness your daughter like the Britax Marathon, Sunshine Kids radian, TruFit, but these seats don't convert to boosters so I wouldn't bother with them. For the money, I would purchase one of the others listed above. I would recommend the Graco Nautilus.
Way to be wise, mama. She's not ready for a booster, a 5pt is always safer as long as possible.