Buy a new seat or not?

I can't decide what to do in DH's car. He can't fit all five girls in his car and they don't ride with him often. He will take the three big girls to school (2 miles away) every once in a while (maybe 4 or 5 times so far this school year). DD#3 rides in a HBB and DDs#1 and 2 ride in nbbs. DD#1 rides in the front seat (the center seat in the back seat doesn't have a shoulder belt...and DD#5's scenera is installed in the center anyway) with the seat all the way back...there is no air bag.

My problem is that occasionally DH will take the morning off so I can go to appointments for myself or DD#2 (I usually take the other girls with me to her appointments, but sometimes I need to be able to give my full attention to the doctor or its an appointment they just shouldn't go to (like EEGs, etc). The big girls are in school, so he just has DDs 4 and 5, but since he's off anyway, he likes to run errands that he usually isn't able to since he's working or something. They don't ride with him often at all...maybe once every 2 months or so. We don't have a harnessed seat that DD#4 fits in that will fit in the car beside the scenera. So do I let her ride in the TB or buy another harnessed seat. We really don't have extra money to spend on a seat that won't be used but a handful of times, but of course, her safety is more important. I have never tried her in a hbb, but she knows the rules. She's heard them preached to DD#3 while we were booster training her. I think she probably would do fine and I know DH would keep a check on her to make sure she was staying in position. But is she big enough (stats in my signature are current)? Should I just buy a harnessed seat or give the hbb a try first?

Is she just turned 4 or closer to 5? She is on the small side for a booster, but she does legally fit in the Turbo and it fits small kids well. If you think she can handle it, maybe try it once to see how she does and then decide from there. Probably not what you wanted to hear, but that's my thought on it.

We started booster training DD shortly after she turned 4. She does really well in a booster.

I would be fine with that, or you could take DHs car and he could drive your car when such need arises.

She's 4.5. She'll be 5 on September 11. I put her in the TB this evening and she's still on the lowest setting, but it fit her well.

I JUST moved DD#3 (who will be 6 on April 11 and finally hit 40 lbs) to a hbb full time and I had a hard time with that decision. DD#4 just seems so little to be in a hbb. If she rode with DH more than a handful of times a year I'd buy a new harnessed seat, no questions asked. I think I'll take a short ride with her tomorrow in the hbb and see how she does in it. If she wiggles, etc, that will make the decision a lot easier.