Hellobee Boards

You don't really install a booster, some have latch just to hold them in place, but many you just set on the seat. Since your DS is still young, he may need to use the belt positioning strap that runs up the back for a backless booster which is pretty much the same no matter which seat you choose.

Personally, I would still want my 4.5 yr old harnessed and would look for a combination seat rather than a HBB. Something that will harness until he's responsible enough for long term booster riding and then can convert to a booster so that you're not buying a new seat. I recommend the Graco Tranzitions/Ways since it is a fairly narrow seat and relatively inexpensive. The AAPs new guidelines are that you keep a child harnessed until the reach the limits of that mode for the seat... but realistically, I think most kids can transition to booster between ages 6 and 7.

Yes, your DS is too small for the Graco Turbobooster to be used backless in any event (it’s for 43”+) and is very young to be in even a HBB. We first used a HBB for travel just after my DD turned 5 (can’t be used on the plane but it was easy to transport and use at our destination and we weren’t driving that much there). It went fine but she’s staying in a harnessed seat for regular use until she’s at least 6. A combination seat is definitely your best bet.

@PurplePeony: I totally missed the height discrepancy! Good call!