Start of 3.x to 4.x upgrade instructions.
Note that unlike most major version upgrades, a minimum version of 3.3 is needed before upgrading to 4.x. If running an earlier version, upgrade to 3.3 or later before upgrading to 4.x.
Doing real-time electronic check transactions? After the upgrade, double check your billing events to make sure the upgrade didn't add a duplicate "Run card or check with a Business::OnlinePayment realtime gateway"