Confused over when to use a booster

I was trying to send those 2 videos about those little girls that died because they were in a booster to young to my sil since her dd 4 in nov has been in one since a little before 3 she out grew the harness and they could not afford a high weight seat and high harness seat. Right now she weighs between 42 and 45 lbs and between 42 and 44 inches tall now.

One video the girl was 3 and 11 months and there only injuries was to her abdomen it did not give weight or height just the age.
So it got me thinking on my dd who is 4 and 3 months 43 to 45 inches tall and 45lbs is it really safe for her to be in a booster if sitting the right way and if the belt hits where its supposed to be even for a emergency? She has a Vivo. She would have been in a booster at age 3 she was 40lbs if there were no high weight seats out.
I cant afford another GN at this time and my husband works nights uninstalling the GN every night was a pain. I would have to uninstall and reinstall the GN about 14 times a week. Shes too tall for the New Evenflo seat Generations 65 seat. If she ever has to ride in the aunts jeep again she will have to be in the booster as well if she also has her cousin with them she in a booster too and the only place the cousin booster fits is in the middle 2nd row. The only spot where my dds GN fits as well. I will try not to let that happen again. For a few more years though.

I know its best to wait till they are closer to 6 for full time booster use but for booster training once or twice a month or emergency's is she really safe at her age weight etc.?

Honestly, while I don't think a 4y/42# child in a highbacked booster is ideal, it's not a situation in which I'd feel the need to intervene -- especially if the family can't afford to do anything differently.

If your daughter can use a booster properly then I think she's okay in oneif she has to be. Since you do own a harnessed seat, though, I'd try to use it even if it's "a pain".