I think you're fighting against geometry. The plates you're using have either an 18.43 degree angle or a 45 degree angle, which have non-integer hypotenuses. The hole for your enclosed 4x4 plate (for example) is sqrt(2)*3 studs (about 4.24 studs), which leaves a 0.24 stud gap (1.94mm), which is only 0.12 studs when placed "centrally".
LEGO doesn't make any "wedge plates" (in BrickLink-speak) that are integer-based right triangles (Pythagorean triples), but it'd be great if they did. In the meanwhile, we're left with with "fudging it". If your hole is (say) 7.98 studs wide, you might be able to squeeze a full 8 studs inside there-- or if it's 8.04 studs wide, you can see if the small gap is objectionable.
I'm not sure if there are any combos that work especially well at the scale you've got. If (say) you were looking to get a full 48x48 baseplate or more rotated at a funny angle, there are a few oddball options that might work. But in the "16-studs-and-under" size, not too much.
There may be other oddball things (like your 2nd option) where there's some space left at the corners, rather than uniformly all around, but I'm not aware of any offhand.
One option for uniform gaps is using the new 2x2 wedge plate:
Using 5 of these on each side (20 total), you can get a 63 degree rotated 11.18 hole that's got a gap of 0.18 studs (0.09 studs when place centrally, which is only marginally better than your 4x4 square)
You can also use these plates:
Using 4 of them, you can get a hole that's 4.12 studs square, at a 76 degree angle. That's not too bad, but it's not a big square to work on.
Another option are these:
That'll get you a 12.17 stud hole, with an 81 degree angle.
Apart from that, other solutions I might work towards would be:
1) Make your base thicker, and use a tiled surface under your angled structures. That allows you to get whatever angle you want, but it does mean removing the underlying white plates from your buildings, and it means using more pieces. Keep a handy chart of the Pythagorean triples handy so that you can "stud-down" your larger buildings with more than just a single stud. (This is usually what I try and do)
2) Cover up the holes. Some strategically located small "snow mounds" scattered all around can hide the holes, and provide a different landscape texture.
3) Put white plates/tiles, or even just white paper underneath the holes. With everything else going on in the scene, I'm not sure the gaps would be all that noticeable-- particularly when they're white.