Because the landlord has not clearly agreed on the spatial coordinates of ABCD, although the relative positions of each point are certain, the final answer will be different. I determined the coordinates of ABCD according to the attached figure, and put point C at the coordinate origin.

AE^2=AB^2+BE^2= 16+BE^2，

fp^2=cd^2+(6-df)^2= 16+(6-df)^2，

Aepf is a parallelogram, ∴AE=FP,

∴ 16+be^2= 16+(6-df)^2，

∵BE=DF,∴BE=DF=3。

∴ The coordinates of point E are (0,2,3), and the coordinates of point F are (4,0,3).

∵BE and DF are parallel and equal, ∴BEFD is a rectangle, ∴EF∥BD,

The angle between ef and AC is equal to that between BD and AC.

∵ABCD is a rectangle, and BD and AC are its two diagonals, so that the intersection of the two diagonals is O and the included angle is φ.

∴ac=√(ab^2+bc^2)=√(4^2+2^2)=2√5,od=oa=ac/2=√5

According to the cosine theorem cos φ = (OA 2+OD 2-AD 2)/2 * OA * OD = (5+5-4)/2 * √ 5 * √ 5 = 3/5 = 0.6,

∴Φ = 53 degree. ?