It really depends on what you consider a major outage. If we set the bar at at events that would break customers using a single hosted location, 4 over two years isn't horrible. Things in this category are fiber cuts, automatic transfer switch failures and other power issues, PoP router mishaps, HVAC failures, etc. Steps can be taken by the provider to reduce the occurrence of these, but some will happen, and customers that care should build around it.
If we set the bar at events that break customers who have taken reasonable steps to be geographically distributed, 4 over two years is horrible. Things here are cascading router failure, BGP misconfiguration or external hijack, major peering dispute, bad push to critical systems. If these aren't rare, it's likely to be avoidable failures caused by the provider. Sometimes the earlier types of problems hit here too --- fiber cuts between regional data centers can be major problems if more things run on that fiber than expected; it's always fun to learn redundant fiber is in the same bundle, and usually that's learned when the bundle is cut.
