[propbeanie]

Had you removed your gun crew and put them back to the repair team prior to exchanging? Is your 4" gun in the same location as before? If you have not left base yet, remove what you did, and make certain your gun crews are off the guns. Save the game, exit and come back in to that career. Try the gun swap again. There should be four slots for the 4" gun, and 2 slots for the 20mm, no matter which of the fore and aft combos you use. If not, back it off again, and try the other combo and see if you get the crew. If you do not, do not leave port. We'll try the "gun fix". Something goes awry in the Save folder apparently, and finding that is more than a bit of a bother. This has been with the game since the get-go. Just remember that before you attempt to do Battle Stations, if you do have crew slots, that you have to Save, Exit and return again for the new gun crew to show at their stations...