4' by 3' is 288*216 feet in scale.
...Pretty tiny, but a great basis for a WW1 frontline: at Vimy Ridge the frontlines were only about 90' apart. (The long axis of the board is a 'more typical low-end' distance for No-Man's-Land on the western front.)
I'd be tempted not to go for an attack dio: much of the time, after all, was spent hanging about, digging, passing the time. Detail attack and support and connecting trenches. The two planes can add interest, one chasing the other over the lines, both being pot-shotted at.
Your Mk1 tank, if used, places the dio after the autumn of 1916. Maybe have it broken down and abandoned (the most likely outcome) in some festering shell hole or other? Indeed, a WW1 diorama would seem to suggest emphasising the awfulness of the environment: broken ground, stumps of trees, pools of foul water in slimey shell holes, masses of wire, dead horses, broken carts, the lot.
Andy