I'm curious if this is an actual port or just the PSX version in some PC emulator?

There was a PC port of BOFIV but was never released in North America. If you see the other thread there are still missing content and stuff still censored. Technically speaking the PS1 with emulation is still currently superior minus the resolution QoLs due to custom patches that can be applied to a PS1 ISO. I am personally waiting for them to add the extra content, more PC graphic improvements before I purchase.

It looks like a port. It has built-in resolution selection and the keyboard keys are the default on-screen. No PSX buttons in sight at all.

missing content? the original PC version had no missing content pal

This is the original PC port that was released in 2003 for Windows XP with all the content from that original release. what people are saying about missing content is. the release censored and removed 3 cutscenes that were deem inappropriate and offensive and changed some dialogue to make things less offensive. some issues from the orignal Port are fixed in this version. but this isn't emulation this is a actual PC port

It is classified as missing content from the original Japanese PS1 perspective. Which Coolhardy pointed out already. Uncensored and removed scenes and the exclusive Dengeki shop. The Legacy thread mentions also the port quality with sounds and graphics differences between the PC and PSX version. So to make it a "definitive" edition would be to restore said content and add other QoL.

And lower the CPU load by using updated stuff that is not made for stone age CPUs.

lol

Technically you get the censored (english) and uncensored (japanese) version, the GOG is the definitive edition so far. You can use kamui to translate the japanese version or be reliant on modders/community to translate the uncensored GOG japanese version into english again.

I wouldn't called it definitive, given the horrible audio loop (music abruptly stops and starts again) and no reverb, which makes it sound flat