Do you see any glaring problems?
Can't tell which direction is UP on the P3 & 4 joysticks.
Search the forum for "angled joysticks"
Most common mistake on 4 player panels due to lots of bad examples online.
I'd also reconsider having the trackball buttons there.
Most people just use P1's button, but if you play a lot of golden tee it's nice to have dedicated and labeled buttons.
For old school 4 way games do I need anymore than three buttons?
The only ones in MAME that use 4 buttons are the couple Dungeons and Dragons game. All the rest use 3 or less.
You could always map a combination to the 4th button in D&D, but I kind of like the look of the design with 4.
Guilty Gear Izuka which is emulated in Demul uses 5 buttons, but it's one game....and not as good as the other games in the Guilty Gear series.
I have a 7 button layout on my cab because I wanted the Neo Geo "four in a row", but I've come to find that I prefer to use the top row and my thumb on the first button of the second row for the "D" button. It just feels more comfortable. The extra button can come in handy on some PC fighting games (steam) if you plan on playing those.
If I had it to do over again, I'd probably either just go with 6.....or up it to 8 to cover all the xbox360 fightpad buttons.