I just took some measurements for you. This is what I got:
- Between the wheel wells is approximately 3'-6 1/2" (42.5"). Above the wheel well is wider but you'd be limited by the hatch width.
- The depth of the cargo area at the floor is approximately 5'-10" (70") with the hatch closed and the rear seats folded down. At the top it's much less. Approximately 4'-5" (53"). These measurements are taken with the seat at my seating position. You could gain a few inches if you are able to pull the seat up.
- The height of the opening is approximately 2'-4 1/2" (28.5") just off center
- Width at the hatch is about 3'-9 1/4" (45.25") max. near the bottom. It gets narrower as you go up.
So you won't be able to get 4' wide lumber in there.
I haven't carried any lumber but I put a 46" Husky workbench in there. The box dimensions were 49.8" W x 28.34" D x 33.19" H (taken from Home Depot's website). With all the curves around the hatch area, it literally just fit with us sliding it in on its side. If it was 1/4" taller it wouldn't have fit. Here are some photos:
This is the workbench in the box:
Hope this helps...