I would go with the Finale over the Maestro for a few reasons, which is why I didn't mention the Maestro as an idea originally even though it is also a good, affordable seat.
The first is a higher harnessed weight limit, 65 lbs vs 50 lbs. Depending on how your kids grow, that might not make a big difference but it could make a huge one. It also has a 10 year expiration date, making it a better deal if you are able to use it longer or pass it down to a possible future child or friend or family. Last reason isn't huge, but it is a bit lighter, maybe not enough to make a big difference with transporting it, but maybe.
Now knowing your 8 year old's stats, forget my suggestion of two Finales, your 8 year old has outgrown it. I'd surely go with a booster for travel for the 8 year old.
I do still think it could be perfect for your 4 year old, for the reasons above as well as when the time comes, it seems to make a fairly decent high backed booster for a bit. The Maestro booster is outgrown at the same time as the harness due to height.
For your 8 year old, I'm still thinking the backless would be an excellent, easy to travel with choice. If you get one soon, you can start doing short trips around town to get used to it and to have your 8 year old practice leaning back into the car's headrest when tired, even if not tired, but practice the position while relaxing, but still staying in place. I would really not want to travel with a high back for an 8 year old, I say that as the mom of a newly 9 year old.
I have been considered quite anal by many friends and family for my crazy car seat use, because it is always so much more than any of them are doing etc. She would be in a high back booster still, but I had to switch to backless because the low slope of our car's roofline made it impossible to have a booster tall enough for her height. She's currently about 56, maybe 56.5" and 76 lbs. Just want you to know that even though I'm not a CPST, I am very passionate about car safety and would never suggest anything I wouldn't feel completely comfortable with myself for my kids. The switch to booster from the Frontier was also due to the car's shortness, booster needed less space at the top, so that worked for a year or so before having to go backless.
Oh, if you do decide you'd be comfortable with your 8 year old using a backless for travel, I'd get either the Harmony Literider (about $13) or the Bubble Bum (I think they run about $30, not sure). They both offer good belt fit, are easy to use, and lightweight. The Bubble Bum is an air cushion with belt guides, it can be deflated and packed very easily and is of course super lightweight and makes an wonderful travel booster.
I apologize for any spelling errors or grammar mistakes, or just general jumping around. I went to bed over 2 hours ago, but couldn't sleep, so I should be sleeping and my thoughts are somewhat random and all over the place, but hopefully I made sense. If not, please ask any questions you may have and I will answer/explain what I meant, when I am fully awake, unless of course someone understood anything that might not make sense and is able to translate prior to when I get back up and on here. I really do hope I've been helpful and that you don't feel like I am dismissing your concerns in any way, I'm truly trying to help you figure out a safe, easy, and not bank breaking plan.