Does anyone know if I can use Preen on garlic that is already about 4 inches tall and also, can I use it on the soil I'm planning to plant both summer and winter squash?
On the garlic, yes.
With squash -- ONLY if you plan to put out reasonably large transplants (say, with 3 or 4 sets of leaves). Preen will inhibit squash germination and proper seedling growth.
You could probably easily start the squash in pots and transplant when large enough.
I have set transplants into Preen-treated soil many times without any problems, AS LONG AS THEY ARE LARGE ENOUGH -- as I said, once they get past about a 3 to 4 sets of leaf stage, the Preen no longer seems to inhibit them.