I have never used SHIII's Mission Editor however, I don't think .mis files made on one ME will work on the other. There are too many different ship/plane files between the two games to have one ME work with both. It's been my impression that when a mission is made, your specific game files (whether they are modified or not) go directly into the mission that's being saved. I've encountered missions I've made using mods that add ships to the game......
not have those ships displayed in-game if I removed the mod. The .mis file expects the same ship make-up to be present when you run the mission in-game. That's why I believe the SHIII .mis mission's will not run in a SH4 game. It's my opinion, you need to chose which ME to use on the basis of which units do I wish to see in-game. SHIII or SH4's?
A couple of things about the SH4 Mission Editor.
I always put a Mission Title to the mission I'm making. Opening the "Missions" menu and clicking the "Language Localization" heading, I will use a name for the mission that I plan to call the Folder which will eventually hold the .mis file in the Data/SingleMissions folder of the game.
In other words, always chose a name for the mission that will be
exactly the same as the folder you plan on saving it in.
After you've made your mission; checked it for faults with the "Missions" menu "Validate Mission" feature, and if all is well....
I will open the "File" menu heading and use "Save (normal mission) as...." to save my created mission. I never use the "Save" heading (I don't know, I just don't!). You'll get a pop-up menu like this (once you have chosen the correct path to save your .mis file in):
I will use the "Create New Folder" to do just that.....create the new folder with the same name as the Mission Title (in this case "AoB Test"). Make sure you have the correct "Type" of mission displayed in the lowest box....in this case an American Mission File [.mis].
When you have the "AoB Test" folder ready to go, then you're ready to name the file "AoB Test.mis", and click the "Save" button.
You should find your new xxx folder, with the same name as the xxx.mis file sitting in the Data/SingleMissions folder. Starting the game, and finding it in the Single Missions menu (or is it the Quick Missions menu of the stock game?) you should be able to chose your new mission.