unfantastic/non-fantastic/not fantastic

[navi tasan]

Which are correct:

1-You think that film is fantastic, but I will tell you how unfantastic it is.
2-You think that film is fantastic, but I will tell you how non-fantastic it is.
3-You think that film is fantastic, but I will tell you how not fantastic it is.
4-You think that film is fantastic, but I will tell you how fantastic it is not.

[Rover_KE]

They are all 'correct' in informal conversation.

A film critic is not likely to put it like that.

Rover

[Raymott]

Which are correct:

1-You think that film is fantastic, but I will tell you how unfantastic it is.
2-You think that film is fantastic, but I will tell you how non-fantastic it is.
3-You think that film is fantastic, but I will tell you how not fantastic it is.
4-You think that film is fantastic, but I will tell you how fantastic it is not.

I'd say, "I'll tell you why it is not so fantastic" - unless you merely intend to assert more strongly that it is not fantastic.