There seems to be a lot of discussions going back to 2005 and here is another one from 2007 on the length of PINs w.r.t. to US banks but here is the thing.
There are ATMs(Cash Machines) that are owned by banks and there are cash machines that are owned by other companies and are installed at any place you can think of the thing is there is no way to know what fractions of Cash machines are limited to 4 digits and which aren't.
Your bank may have relationships with other banks outside home country like Bank of America in the US if you look at international locations. On top of this it may have branches in those countries, so you can be reasonably sure that ATMs in your bank's "Global ATM Alliance" or their overseas branches will be able to accept 4+ digit pin.
If you come up on a standalone ATM it usually states who owns or runs them like ATMs in Duane Reade Pharmacies are run by Chase, which is as if you are going to a Chase bank branch and are unlikely to have any problems with long PINs, others may be some private companies that just placed it in the corner stores and potentially not accept any longer then 4 digits.
Also if you read the discussions I linked you can see that in some cases just first 4 digits may be enough to identify you to the ATM, so you may want to check with your bank, whether this type of scenario will work in your case.