At nearly 5 years old, 2 inches of harness height could be anywhere from about a year to 3 years or so..chances are you'll get 1.5-2 year out of 2 inches though, on average.
There are really a couple of issues. First, your child mitght not need many more years of harnessing. Once a child is old enough and mature enough to sit properly in a booster seat, there is no evidence that continuing to use a harness seat is any safer than a booster. Most kids hit the required maturity level sometime between 4.5 and 6.5 years old, the majority by about age 6. So realistically, you probably only *need* anoither year, year and a half of harnessing, which means that either the radian or nautilus or frontier would give him plenty of harness time by height.
The issue with buying a harness-2-booster combo seat is that it can be hard to know whether ornot the booster portion is going to fit your child correctly. If you can, go try out the frontier and nautilus, and look very carefully at how each seat fits in booster mode. The issue some people have with the nautilus is that it doesn't position the lapbelt correctly, it lets the lap belt ride too high. This can be an issue in any booster, but it seems to happen more frequently in the nautilus than in some others. It happens much less in the frontier, but it still could happen. So, what happens is, you run the risk of buying the combo seat, and you use the harness part, and then when it is time to booster, if thebooster doesn't fit well, you STILL need to buy a dedicated booster.
I have a nautilus, radian, and frontier, LOL. I will say that in MY vehicle, with MY child, we love the nauti hands down as a harnessed seat. My dd LOVES it, and we use it daily. However, when it comes time to booster, she won't be using it. The seat belt doesn't fit right the way teh seat belts lie in my car, the angle allows it to ride up too much on her abdomen. The frontier, she HATES. Its a personal thing, she doesn't like the way it "feels". So, we don't use it much as a harnessed seat. However, the belt fit when used as a booster is good. In a couple years, this might very well be her primary booster seat. The radian she fits into harnessed, and says she likes it, but she has only ever sat in it, it's her brothers seat. She also has a graco turbobooster, which is an economical basic booster seat, BUT..it fits her REALLY well, places teh belts exactly where they need to be, and is therefore an EXCELLENT choice of booster for her.
What seat has he just outgrown?
Realistically, any of those seats would probably be a good choice. Another radian will have plenty of harness height, and you can get a booster later. A nauti will work well in harness mode, and *might* be a good booster later, or could be handed down as a harness seat. The frontier is likely to be both a good harness seat and good booster.
Let me ask you this. How is his maturity currently?? At almost 5 years old, if he has normal (or better) maturity, it would be compeltely appropriate to buy a booster now as a secondary seat, in order to start "booster training" him. If you feel he can not use a booster correctly at this time, then one o fthe other options is probably the better bet.