Your dd probably won't reach 40 pounds before age 4, and as long as she is mature enough for a booster at 4 it's okay. Still, I don't discourage folks from extended harnessing, it's what we are choosing for our own child.
However, as you've found, higher weight limit harnessed seats are few and far between, and somewhat pricey.
The only seats that harness above 40 pounds are:
Britax Marathon, Decathlon, Boulevard (rear-faces to 33 pounds, forward-faces to 65 pounds OR until the shoulders go over the 17 inch top slots, generally around age 5-6 and 45-55 pounds, has built-in lockoffs eliminating the need for metal locking clips with older vehicles/seatbelts, has a rear-facing top tether)
Britax Regent (forward-facing only, harnessed to 80 pounds, 21-22 inch top slots, usually outgrown around age 8-10)
Fisher Price Safe Voyage Deluxe (rear-faces to 33 pounds, forward-faces to 55 pounds, 17 inch top slots, built by Britax and is almost the same as the Marathon, without the rear-facing tether and built-in lockoffs, will last to about age 5-6)
Sunshine Kids Radian (rear-faces to 33 pounds, forward-faces to 65 pounds, 18 inch top slots, rear-facing top tether, narrowest higher weight limit seat on the market but has installation issues with some vehicles)
Safety 1st/Cosco Apex (forward-facing only 22-65 pounds, 17.25 inch top slots, converts to a highback booster once the harness is outgrown, it's a very wide seat, only about an inch narrower than a Britax Regent, must be used with vehicle seatback or headrest at all times at least up to the tips of the child's ears)
SafeGuard child seat (harnesses to 65 pounds with 19 inch top slots, don't know much about it because it's $400 and I don't know many who can justify that cost for a forward-facing only seat that doesn't even convert to a booster and only harnesses to 65 pounds)
SafeGuard Go (more of a travel seat, IMO, must use a top tether at all times, top slot height of about 17 inches, harnesses to 60 pounds and then becomes a backless booster that can utilize LATCH)
You could also look at any of the 40 pound harnessed weight limit combination seats by Graco or Evenflo. Only Graco and Evenflo make combination seats with top slots of a decent height. The ones made by Cosco/Eddie Bauer/Safety 1st all have top slots measuring about 14.5 inches. Nowhere near high enough to get kids to 40 pounds before they get too tall for the harness.
Good 40 pound weight limit combo seats are the Graco Ultra and Platinum CarGo and the Evenflo Chase, Bolero and Generations. These will get your dd to 40 pounds harnessed and then convert to a booster. Only the Evenflo Bolero and Generations will work well as a long-term booster because only they adjust taller in booster mode, and they have open shoulderbelt guides so the shoulderbelt can slide freely.
If your vehicle has high seatbacks or headrests I'd go with an Apex because you are on a budget and want to be able to harness your dd past 40 pounds. Ideally though, I'd get a Sunshine Kids Radian (if you can try before you buy or order online through Target.com and return in-store if it doesn't fit) because it has the highest top slots of all the 65 pound weight limit seats and is a very high-quality seat (just has some installation issues in some vehicles). The Fisher Price Safe Voyage Deluxe would also work well for your dd for quite some time, and is in the middle, price-wise, of the Apex and Radian. It sits up high on a base like most convertibles, so that may or may not be something you like. The Apex and Radian sit flat on the vehicle seat, more like a booster.
If you can afford it and really want to harness as long as possible, go for a Regent. It is big, but in my experience installation is a breeze in almost every vehicle, even small two-door cars.