When you sync the devices to the Mac, you'd want to Store->Transfer Purchases to get all the apps on your devices into the Mac..now that doesn't apply to the music and you will get a warning that your device is synced with another library, and syncing with the new (possibly empty) library will erase your music and so on...
So you might want to first migrate your music library over to the Mac and THEN sync so that you don't end up losing anything along the way..
Why do you have the iPhone 4 and iPad on separate accounts? You could just selectively sync items to either device based on what you want..
I have my iPhone 4, iPad 2, iPod Nano all sync'ing to my iMac, but they have different settings, so just the right things end up on each device, i.e., the iPod Nano sync's to a specific playlist, the iPhone 4 has no music at all, just apps and a couple of videos. The iPad 2 has it's own set of apps and videos and books that it syncs with..