Type 3 Perazzi guns can be identified by the forearm attachment to the iron. The type 4 has a screw on either side of the iron which tighten the forearm against the iron while the type 3 does not. I am sure that there is a difference in the the barrel lug for for forearm attachment as well.

If some one would post a photo for me I have an image from the net that makes the difference very clear.

jlb