I am hoping to upgrade to 4 soon. Once my new theme is completed I want to make the leap over to 4. I've been testing/learning about 4 on a dev site for several weeks now. Aside from a full backup, what other things should I do prior to upgrading? Any gotchas I should look out for? What about mods/apps? Any issues there?

Thanks for any feedback you can provide!

Before upgrading, I would suggest make sure that your 3.x instance is as clean as possible. No errors, database issues, uninstall 3rd party items that do not contain crucial data, etc... A lot of issues stem from that. Kind of dummy statement but ensure all IPS4 files overwrite your 3.x files and are on your server before starting. 

Those are usually where a lot of problems stem from. Good luck! Let us know if you need help  

all your apps and mods must be 4.0 if you want to get the site to works other way you have to delete them after migrating (and lose data associated) so the only thing i can tell you is wait for th apps to be ported before migrating

The only mods I really care about are:

• Member Maps
• Links Directory
• Videos System

If I uninstall these, won't I loose the data I need after the upgrade?

The only mods I really care about are:

• Member Maps
• Links Directory
• Videos System

If I uninstall these, won't I loose the data I need after the upgrade?

Links Directory could be done via Pages as for Members Map as only using Google API and Video System is that same as https://community.invisionpower.com/topic/416243-videos-support/

This might be of some help.