I got my EX4 in February and my first plug went in July, the second is Aug and 3rd in early September, each time weber replaced them free of charge with no questions asked. On the last time they sent me 3 replacements and also a replacement holder, stating that I'm going through them pretty quickly. This seems to have helped, the new holder looks exactly the same, and the original did feel like it was connecting well, but the plug does seat much firmer on the new holder so it is making a better connection.
I use my grill regularly and have had no issues since.
Weber rep also told me that they are aware that the glow plugs aren't lasting as long as they should and they are looking at the issue.