Want to push more buttons but can't because of your 4 button limit? Don't want to spend a lot of money/any if you have a second keyboard. I have a simple alternative then!
All you need is 2 keyboards
a ps/2 to usb or DIN to ps/2 to usb if you have older keyboards
Make sure your computer has enough slots for both keyboards. I have 1 USB and 1 Ps/2 so I didn't have any troubles.
This is great for fighting and older games! Especially when you need to aim and shoot or jump move and shoot etc. Or even if you want to synthesize a piano or keyboard. There's many uses.
Step 1: Step 1
Remove all the keys you won't be using. I removed all but the arrow keys onwards then decided I wanted more room tor the mouse so I removed all but the number pad. You could also do it the opposite way so you're using W A S D on the bottom keyboard. Give it a good clean if's not already, I posted some before shots of the keyboard, not a pretty sight.
Step 2: Step 2
Then all that there's left to do is to attach it to the other keyboard to the main one. I just used some bluetak and a a piece of wood on the hanging side to keep it balanced. I'm sure there's plenty you could do here to make it look nicer but I was just doing this as a quick solution.
Step 3: Finished!
And you're done, you should get something that looks like this or this, or better if you put more effort into it.
I also have another idea for doing it in one keyboard but making a little box for multiple controllers.