CyanogenMod developers have said that they have already started working on the recently open sourced Android Ice Cream Sandwich edition.
They are aiming to come out with builds based on this edition in two months. So, you should be able to get Android 4 for your older devices starting January.
Samsung Galaxy Nexus is the first smartphone to feature Android Ice Cream Sandwich and it goes on sale this month.
And Google as promised released the source code just days after the actual launch.
It remains to be seen how soon we would see custom builds based on it powering older Android smartphones. Manufacturers take a lot more time to release their own editions and they tend to skip a lot of older devices while doing so.