This is trial and error -
1. A square of side 1 has sum of diagonals 2 srqt(2) = 2.8+.
2. A flattened square (think parallelogram of side 1 with 2 parallel sides both nearly on the x-axis) has 1 diagonal of length 2 and the other of length 0, so sum is 2.
3. A rectangle of sides (2-x,x,2-x,x), with x just a little bit larger than 0 has diagonals of about 2 and 2, so the sum is 4.
So I'd guess the maximum length of 2 diagonals of a quadrilateral of perimeter 4 is 4.