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It is probable. I can't find perfect specifications, but the dimensions of the Pro 3 and Pro 4 are very similar. Lining up the machines visually, as shown in the image linked below, indicates that the dock port is in the same place and is the same size. That's by no means proof but it's not ruled out. The USB and DisplayPort certainly change by comparison.

I have ~20 Surface Pro 3's in production so I am very interested in this answer.

Surface Pro 4: 11.5" x 7.93" x 0.33"

Surface Pro 3: 11.5" x 7.93" x 0.36"

http://i.imgur.com/aUCyzLi.gif

Again, this is a sloppy image comparison, not a guarantee.

I've heard rumors that they'll release some sorta spacer that will allow the pro 4 to work with the old docks.

yes[1]. I thought I read that they promised this at the surface pro 3 launch but I can't find proof of that.

[1] http://www.expertreviews.co.uk/tablets/1403174/microsoft-sur...

Pretty sure I heard the presenter say 'surface pro dock, yes I said surface pro not surface pro 4 because the 3 works on it, too'. Should double check though.

I've heard this detail but I am curious about the other way around.

The simple answer is, yes.