Question

In quadrilateral ABCD, we have AB=3, BC=6, CD=4, and DA=4.

Answer 1

The triangle inequality applies.

In order for ACD to be a triangle, AC must lie between CD-DA=0 and CD+DA=8.

In order for ABD to be a triangle, AC must lie between BC-AB=3 and BC+AB=9.

The values common to both these restrictions are numbers between 3 and 8. Assuming we don't want the diagonal to be coincident with any sides, its integer length will be one of ...
  {4, 5, 6, 7}