All C4 axes will have a coincidental C2 axis...only because a C4 axis implies that rotations of 90°, 180°, 270°, and 360° result in no change in the structure's orientation. To be a C2 axis, this only has to be satisfied by rotations of 180° and 360° - so a C4 is a special kind of C2.
Like you said, a rectangle has a C2, and a square has a C4 - but the square also has a C2 - after all, a square is just a special kind of rectangle. A C4 is a special kind of C2.
And P.S., a C4 also has a coincidental C1
And P.P.S., A C8, you may have guessed, has a coincidental C4, C2, and C1.
Does it mean that the XeF4 is a rectangular base rather than square base?
Not quite...and I hope it is clear after what I've said...if it's not, let us know...I've been off of my game for a while