How long ago was your experience? This may explain why (over the past 5-10 years or so) I've encountered more and more machines that require to press OK after entering the four digits (before, some machines would submit after you type the fourth, without confirmation).
Not really. You'll break international standards if you do. I used an 8-digit PIN for many years, but it is 100% impossible to use anywhere I tried in Europe. You will be completely without any way to pay other than cash. No machine will allow more than 4 digits in my experience.