so what to mine after eth reach 4gb dag? what other coin can be mined

Until 4 GB (4,000,000,000 B) < ETH DAG Size < 4GiB (4,294,967,296 B), the answer is ETH.
That's not how it works. Any OS, even Linux, is going to use some of the 4GB of VRAM. If you're running Win 10 you're looking at a few epochs left before ETH is no longer an option.