The upgrader to v4 all it says is The database has encountered a problem nothing else , rechecked v3 settings several times , they are all correct page is cached so once you see this , you cant see teh importer anymore unless you clear cache , 3.2.4PRO, v4 4.2.9 "stable"
You do not need to clear cache, just logout then login again to admin cp. Please look for detailed error information at aMEmber CP -> Logs -> Error log