IF I had to do it all over again here's what I would do:
I went to Northwestern and nearby was Oakton Community College, which has a PTA program. I would have taken a light load at NU (not changed my major 4 times), and instead of graduating a bit early from undergrad, I would take the full 4 years at NU, but have completed the PTA program in the process. Depending on the time I had, I'd take prereqs at either Oakton or NU.
If you're majoring in something unrelated to PT (not a science, but something like dance, communications, etc.), then take the prereqs at the community college if you can (some of them like A&P and bio you'll need for PTA anyway).
I would not quit the university though if you don't have to, especially because some of the PTA classes won't transfer to a 4-yr school. Instead transfer the gen eds you take at the 4 year to the community college for the PTA degree (that way you won't have to take them twice).
Then after you get your PTA degree, you can work part-time while finishing your undergraduate work. Then you have a fail-safe plan if you don't get into PT school right away.
If you get the PTA first, and then try to transfer to a 4-year school, you may be less motivated to finish the BS. Think about it: if you get a PTA degree after 2 years, are you going to want to start taking classes again at a university, paying more tuition, or are you going to want to work a pretty decent paying job? If you keep taking classes toward your BS though while you're doing the PTA, you're more likely to finish it (since you've already put in the time and money).
You'd have to check though to see if scheduling is possible. At NU and Oakton fortunately the classes wouldn't have conflicted, or I could have found a way to do so...that might not be the case though for you.