If you ever get up to the cities, somewhere in the metro area are some bald cypress trees (Taxodium distichum) that I believe produce viable seed. I am not sure where they are, but I think someone on the forum does. They are on private property.

There is a Pseudolarix by the Lake Harriet rose garden that I have seen cones on. Being a lone tree, I am not sure if it produces viable seeds. There is also a huge Magnolia acuminata, while not really pushing zone 4, it is a tree that is rarely planted.

The Saint Paul campus of the UofM has some unusual trees scattered about. Again they may not all be pushing zone 4, they are rarely planted. Acer triflorum, Acer tegmentosum, Magnolia tripetala, Sophora japonicum, Pinus koreanis, Cornus mas to name a few.

Another worth looking for if you can find a local tree producing seed is tulip poplar (Liriodendron tulipifera), not actually a poplar. The UofM arboretum has some mature ones.