Threaded inserts

[plargen]

i think you would only manage a m3 insert as the hole required for an m4 is about 6mm and they have a thread depth of about 1.5-2mm depending on which 1 you go for

http://www.ebay.co.uk/itm/M3-x-6mm-S...item3f21b515c8

also there are a push in type specifically for plastic but from what ive see they pull out after a bit off abuse