You can go ahead and sew section 3 onto your completed sections 1&2, and you can also attach section 4, but you need to leave it partially undone. Yes, it's another partial seam, but they're nothing to be afraid of remember :-)
Here is a picture of my GW with sections 1,2 and 3 sewn together with section 4 lying in place at the bottom. We need to attach section 4 so that next month we can attach section 5 (simple seam), then later section 6 (simple seam), then finally section 7. Section 7 will be attached to sections 4&6, then across the bottom of section 2 (hence needing our partial seam). If that all seems confusing, don't worry, I'll post plenty of photos along the way.
For now, we're going to sew section four onto the bottom of the block that consists of sections 1,2 and 3. As far as I can tell, the HST block should overlap that square in square block you can see by about a 1/4". We're not going to sew all the way to the end though, just to that first orange pin.
So there it is, attached most of the way across, with a couple of inches of open seam at the end, so we can attach the other parts later.
If you are making speedy progress with this section (and maybe even section 5), then make a start on section 6. It has a lot of blocks in it so will keep you well occupied :-)