For 4096 I use a bleed margin of at least 8 pixels. Might be overkill but better too much than too little. For all I know 8 might be too little lol. Seems to work though. I think actually the bleed margin settings in mudbox are multiplication, have to check that. In any case my margins are pretty big. Hang on I'll show you a margin on my above head texture WIP with a template overlay. Let me go to my computer.