No, your mechanic is bullshitting you. The head does not need to be removed. This is a fairly common issue with an even more common fix (in fact, this is what the dealer would do to fix it). What you want to do is install a Timesert thread insert (find the kit that is for 4.6/5.4 Triton motors) and it will be good as new. I don't know of any specific retailers, but pretty sure it can be found at amazon.com and I've heard they run around $300. Expensive, but better than your mechanic's proposal and is a reasonably simple diy job. The shittiest part is that it was #4 that blew out; that's the most difficult one to get access to, but I have faith that you will overcome it.
2001 GT Convertible
8 cylinders, 5 gears, red.
6 cylinders, 3 gears, also red.
Remember my motto kids: Be bitter, stay angry and blame everyone. It's on my fridge magnet!