Quoting Chiara Leigh:" Around Christmas my son was hospitalized due to RSV. He slept fine before & during hospital stay. Once ... [snip!] ... be 5 months old in a few days. Is it possible this could of been the 4 month sleep regression or something else entirely?"
Could have been...or it could have been teething, or just the fact that his routine was messed up and he was struggling to get it back to normal. Even now at 7 months, if we put DS anywhere to sleep for the night that isn't his crib (like a playpen if we're travelling) it takes a few days to a week to get him sleeping through the night like he used to do. Minor disruptions to a baby's (at least mine) sleeping habit can be disasterous for a while. I no longer look forward to vacations...haha