Actually, unofficially, the older iMacs support 6 GB (4+2) since the first aluminium iMac (mid 2007, I think)

I bought a 4 GB DDR2 DIMM for my Dad's mid 2007 iMac, installed it together with a 2 GB DIMM and it has been running non-stop for almost a year now, without any problems. DDR2 is now a bit expensive, due to being old, the 4 GB DIMM cost me around 50$ on eBay.