#3 can and #4 cannot form a triangle.
Why? For a triangle to be formed, two shortest lengths of a triangle must be greater than the longest length, or else all three lengths cannot connect. It makes most sense when you visualize it.
For #3, 1/4 + 1/3 > 1/2
For #4, .01 + .02 < .205, therefore cannot form a triangle.
