My advice is don't do it.
A Shinobi cabinet isn't big enough for a 4 player control panel and the monitor isn't angled back properly for the players all to see it, and if you graft a panel that big onto it you are likely to make it a flip over hazard. Even if you avoid making it a flip hazard it will look ridiculous and player 3 and 4 will probably have a very hard time seeing the screen.
Your shinobi probably looks something like this.

While almost every non-showcase 4 player machine looks something like this. They don't all follow that basic design scheme by accident, it is the only one that really works aside from a giant showcase cabinet.

When you try to turn a 2 player machine into a 4 player machine the result generally comes out looking something like this (and this one avoided the flip hazard, most make the giant panel 2 feet deep to boot).
