It really depends.
Most 4.5 year olds aren't ready even to try a booster, but sitting next to a parent for a short ride may be a special situation. If you have a very well-behaved child who won't try to unbuckle on plane or car and are right there, as long as you're getting a good fit when you try it at home I'd be ok with it.
I absolutely wouldn't use a Mifold. It frequently causes unsafe belt fit and the fit varies so much from car to car that without checking beforehand there's no way to know it will be safe (and good odds it won't.)
The Bubblebum is a reasonable choice though many children have more difficulty balancing on it than a traditional hard booster- booster riding takes practice, and it's not the easiest one to start with.
The Harmony Youth Booster is a small lightweight option that might be easy to carry. You could also have it shipped ahead to your friend if they'll b meeting you.
Other things to consider: will she be in the car when she might be very tired? If so it may be difficult to keep her upright when she falls asleep. Will the car have a lap-shoulder belt and a headrest center (or tall enough back for her plus booster?) Does the vehicle they have provide safe belt geometry for a booster rider (a few don't.) Will she fit the plane belt (depends what airline likely- she would certainly fit an Alaska belt but may not on some of Delta and American's planes where the belts simply don't tighten as much.)
If you decide a booster isn't a good choice, though they are more expensive you could look at a Ride Safer Travel Vest.
Of course, the SecureKid is a great option for travel. Lightweight, fairly narrow. You could get a cart to lug it on or use a Traveling Toddler strap; that eliminates all worry about appropriate belt fit etc. (Just make sure she isn't over 50 lbs at the time if you want to use lower anchors- if she is, or you're installing in a spot without them, you'll need to use seatbelt and top tether instead.)
It's a trip to Disneyland that we aren't planning ourselves. So, we'll fly down, get picked up by the friend and taken to the hotel (Orange County to hotel, not sure how far that is). I don't know what hotel we are staying at, so I don't know what mode of transportation we will have to take to get to the parks. I'm hoping it's walking distance. If it's not, I'm not sure if he will pick us up and drop us off or if we'll take Uber or taxi or some other mode (so that would be the second need for a seat). If we have to do that, I'm looking for something we can use in those situations that we can stuff in a backpack (ideally...which is why the mifold jumped out at me). Then the second day we're down there, we will be transported by the friend to some other activity (I don't know what that is yet). Then a 3rd day we're going to Disney again.
We always fly Southwest, which is what they booked for us, and last time we flew, the belt fit her just fine. We usually fly with the SecureKid and have a GoGoBabyz cart, so that's not really the problem, I'm just looking to lighten our load and streamline the seat choice.
I guess the answer is to wait and see what hotel they put is in. The only time it's going to matter is if we have to take an alternate vehicle (like Uber or whatever) to the parks and stash it with us around the parks. If that isn't going to happen, then I guess the worry is all for nothing. Although I also just remembered they have lockers or something, right? So we could put a traditional hard booster in a locker...do those ever become full and unavailable?
I have plenty of time to think this through.