So I was trying to play Doom 4 on my PC on medium settings, and the game runs at, at most, 20 fps. I've been wondering if my PC has become underpowered with the current market. Specially when I see games like RE7 needing for its minimum settings more than what I already have.

Do you think this setup is okay, or is it somewhat dated? Or do you think my framerate problem with Doom has nothing to do with it?

Windows 8.1
RAM 4gb
Processor Intel Core i3-4330 CPU 3.50GH
NVIDIA GeForce GTX 750

EDIT

Okay, so I've been looking for some pieces, and I wanted to ask yow what you think about this possible setting:

Windows 8.1 Pro 64 bits
EVGA GeForce GTX 1060 SC Gaming 6GB GDDR5
RAM. 16 gb
Processor Intel Core i3-4330 CPU 3.50GH

I think its the RAM that is the biggest issue. You should toss another stick in there.

You seriously lack RAM (need 8GB) and your video card can barely cut it at those settings, so yeah, time to add and upgrade.

Nah its fine.
As long as you are okay playing sub 1080p / max settings.
Try going down to like 1600x900 or so and medium/high settings instead.

4GB system memory isnt much though.... windows can use like 1.7-2.0 GB alone when your not doing anything.
Then start playing a game and you only have 2 gb for the game? its a bottleneck for you I suspect.

At this point its honestly not acceptable to have less than 8gb system ram, if you plan to game on it.

The good news is memory is crazy cheap :) wont cost you much to solve this issue.

I'm wondering how your PC still runs at all.

You should check your CPU performance and RAM usage during play to see where the bottleneck is.

Son, it's time to upgrade.

Go get 16GB of RAM at least & Geforce GTX 1060 3GB

https://www.amazon.com/ZOTAC-GeForce-Compact-Graphics-ZT-P10610A-10L/dp/B01KKJAJM4/ref=sr_1_2?ie=UTF8&qid=1474886790&sr=8-2&keywords=Geforce+GTX+1060+3gb

750 is not a gamer video card, it's meant for small games and e-sport games.

A PC is outdated fast when you bought a PC with a low-end graphics card to begin with, my 3 year old PC is still up and kicking, since I decided to buy a fast i7 with a 780GTX, which still runs everything.

Spent more, upgrade less.

But I bought my 750 when it came out, it was top of the line :(

GTX750 was never top of the line, a gtx 570 or possibly even a gtx 470 are more powerful and came out several years earlier.

Nvidia cards ending in 70/80 are top, 60 is high-mid and 50's are low-mid.

The GPU is your biggest problem, though you'll need 8gb of ram for modern games too, CPU should be mostly fine.