I don't think moving the bulkhead 4" will cause any strength problems at all. The issues will all be what you have already discovered - what else has to change due to the bulkhead change. 

You might even turn this mishap into s feature. Many commercial kayaks have small area behind the cockpit with a 'day hatch' that can be opened while under way, with no risk of filling a major floatation area of the hull. Do a second full bulkhead in the proper position and add a 4" port on one side of the deck (easier to access). This water tight area is now a 'day hatch and storage area' that can be opened while out on the water.

The Petrel SG (Stich and Glue) has such a day hatch, although it looks a bit bigger than 4".

Good luck - and be inventive!

Joel

I explained in the other post about this situation but regarding the size.  In my mind, 4 inches seems so small.  Is it a decent size to use still? The one I am looking at has a red bag that you can removed in order to get to your belonings.  But I would think if you try to plull the bag that it would get cought buy the items inside if they don't line up right as they try to pass the opening.  Though 4 inches would work better in size regarding where it may fit on my kayak so........