"After four weeks away, his wife was happy to see him back" or "Hi wife was happy to see him back after his four weeks travelling at sea" ('his wife was happy to see him back' is very shorty and simple, so if it's the second part, it's strange if the firs part is long)

First sentence is incorrect. Change "on" to "at" would make it ok ("After he travelled four weeks at sea") Second is ok, but I would make a small change to make it sound better when you speak. ("After his four weeks of sea travelling"