How many valid resonance forms can be constructed for aniline?
Your four structures are correct and you could also include your suggested fifth structure (your first and fifth structures are comparable to the resonance structures of benzene). While some of the structures are mirror images, we generally think of particular atoms as being fixed when writing out resonance structures. You can think of the carbons as having a number (which they technically would if you named the resonance structures IUPAC style), which leads to the "mirrored" structures not being identical since the double bonds are between different carbons.