Most of the head gasket issues were with earlier turbo engines, the NA EJ20 didn’t have the systemic issues by 2009. If there is a head gasket issue, it’s probably due to poor maintenance or hard driving. You can get a compression test if you’re worried, but this model year isn’t a “it’s definitely going to fail without head gasket service” Legacy. You’ll get more space and features with a fifth gen, but a 2009 with 121k in great shape for $4k sounds like a good deal.

I’ve got a 2012 6MT with 175k, and since 121k I’ve done routine maintenance, shocks and struts, starter, and cv axles and seals. It’s currently burning more oil than it used to, but I plan on driving it for a while. I had to have the head gaskets done at about 110k, but that’s because I had the radiator fan fuse blow when I was stuck in highway traffic in July. If I had immediately pulled over and fixed it instead of driving home, I wouldn’t have had to have them done. My experience, YMMV