I want to know if there is a way to insert a 3" X 4" fillable stamp/form onto a page of an existing PDF.

In addition to Thom's correct suggestion you can also use a dynamic stamp. That's a stamp that contains form fields which can be filled in the moment it's applied (but can't be edited afterwards). Thom actually wrote a whole book about this complex subject. I suggest seeking it out if you want to use this nifty feature.

You mean that you want the stamp to contain fields that are fillable after it is applied. The answer is NO, PDF stamps can't do this.

But you have another options, page templates, which can do this!!