I am replacing a 3 ft single pane window with a 4 ft double pane window. The old header was just a single 2x4 laid flat with 2x4 cripple studs spaced 16 inches. My question is: can I install a single 4x6 beam (douglas fir #2 grade) instead of sandwiching two 2x6's with plywood? The charts that I found in the IRC list only 2x's.
In my experience, and assuming your lumber doesn't have major checking, a 4x6 is adequate as a substitute for doubled 2x6s. Whether it strictly meets code is another question, but I'd assume that it does.
It'll almost certainly be adequate in your specific case, where no structural header existed in the first place.
FYI, you don't have to sandwich plywood. You can use a flat 2x4 bottom and stand 2x6 sides on that in a U configuration. That's how I've done it for many years. In the case of 2x6 exterior walls you can insulate between the side members.
Yes, a 4x6 Douglas fir #2 is fine, even with the wider 4' opening. Here, we use either 2-2x header or 1-2x header that is up-sized to the next larger size.
The 2-2x header is more stable than a 4x. As wood dries out it will twist and could crack the wallboard, especially at the ends. We use "kd" or "surface dry" (not sure you can find it where you live). Surface dry is stickered and allowed to air dry for weeks. "Kd" lumber is put in a kiln and dried down to 15%. This removes most of the moisture so it doesn't twist much when it dries out over time.
By using a single 2x8 header you can fill the rest of the wall thickness with rigid insulation. One problem with this system is that drapery hardware cannot be screwed over the header. If this is a problem, we add 1/2" plywood on the inside and reduce the thickness of the rigid insulation accordingly.
By the way, we apply the header on a 2x flat and apply another 2x flat on top of the header. The one on the bottom is used to secure the trim to the surround and the one on top is used to help hold the header straight and true.
No matter which system you use, remember, do not the nail the head of the window frame to the wood header. The header needs to "flex" from snow loads, etc.