GTM Sycnchronization group between Different physcial boxes

Yes, it is possible to have different HW version (including VE platforms) for the same GTM sync group. All members must be running the same SW version and other (see https://support.f5.com/csp/article/K13734). For different SW version is possible run gtm_add script for 'manual sync' but it is not very clear solution.

You can also combine two 1600 and two 4000 platforms. It depends on expected load. For example 1600 has approximately up to 200.000 req per second (RPS), 4000 up to 400,000RPS. for more details see data sheet: https://www.f5.com/pdf/products/big-ip-dns-datasheet.pdf

it always depends on design. maybe better solution (depends on your situation) is reserve two 1600 for BIG-IP DNS, and more powerful 4000 for LTM. it's up to you.