Can I login to my PSN Account on a console from a different region and purchase DLC, and PS+ using that account?
It... is complicated.
The console has the ability to play game disks from any region. So, if you for example buy a Japanese game because the company decided that "no physical release for west market the game will work fine. An you will be able to receive updates too without any issue. I have personally experienced that with some Hatsune Miku games that at the time were unavailable outside japan.
Problems start when DLC are involved.
Since corporate capitalism has to empty you pockets as much as possible, they don't actually want you to be able to use other region marketplaces freely... "licensing issues" being the main excuse. This means that while any region game will work on any PS console, DLC for that game must be downloaded from the appropriate region.
Simpler version: if you buy the Japanese physical deluxe version of a game only available digital in the rest of the world, any DLC code included will be useless unless you have a Japanese PSN store account too. And if you buy something from a different region store you then have to use that store to buy any DLC too, even if the game is later made available in your region.
Is this stupid? Yep. But you will have to live with it. Because in 2022, with every company praising how "digital is better because it means we can deliver you content everywhere fast"... you are still bound to regional markets because some corporate CEO needs to make you pay "import scam" anyway...
Generally, your PS4 gaming console is not region-locked. In addition to the console, PS4 game discs and digital content are not regions locked, either. However, Sony recommends that you get discs and consoles from the same region.