Framing questions for "sun room"

I’m building a "sun room" on a 10×40 foot pad on the back side of my house, with 2/12 roof tied in to house roof. Have built a concrete block wall 24" high, will go up from there.
To allow for the roof slope, the total wall height will be 76" at the low side, so framing will only be 52".

In order to get the most window space, is there any reason I cannot use a 4×6 as header/top plate?

I believe I have it all worked out except for the best method of tying walls together at the corner – notch the 4×6 horizontally? or vertically?

Also how much support is adequate between windows – 3 2x4s?

Thanks!