I've given this some thought - not that my thoughts actually count. Yes, it should have 4 shoulder buttons, 2 per side.
Right now we're using a big flap paddle that also acts as the exposed corner of the device. Let's not repeat that, OK?
The shoulder or trigger buttons should be able to be used individually (selectively) or mashed (all at once) or one held and the others hit selectively. That leads to an engineering issue since we're really talking about something that is only going to be used with one finger per 2 buttons.
I think I have a solution. Two closely spaced switches with tall narrow caps. They could be upwards of 4mm 'long' left to right, but only 2-3mm from peak to peak vertically.
/\_/\
And that is just about to scale. The points could be rounded off a bit.
It would be easy to feel which button you're on. You could rest your finger on the valley between. Push down and in for the lower, up and in for the upper, or straight in for both.
Think of it as a finger-sized rocker switch with a hinge/split in the middle.