So, in my opinion, if you're going to continue supporting MyBB 1.6 and MyBB 1.4 - what about having a custom field for users to select their MyBB version (rough - 1.4, 1.6); and then when you go to the mods site, it shows you only mods/plugins/resources, etc for that version (the one you specified)? Just a thought I had.
While the default categories are only showing submissions for 1.6, downloads for 1.4 are still available in the archive section. You can expect to see some improvement in this area with the release of the new mods site in the coming months.
http://mods.mybb.com/archive/10
I'm just saying, as a suggestion, if a user inputs their MyBB version on the community forums, it would be relevant to use that version on here on the ideas site - to show relevant mods/plugins, themes, resources, etc. And maybe a message or something saying: "You are currently viewing MyBB <VERSION> <CURRENT_PAGE>. Click [url=]here[/url] to view <OTHER_VERSION> <CURRENT_PAGE>" (mods, plugins, themes, resources).