It's hard to say for sure if there will be a Thor 5, as it depends on a number of factors. The most important factor is probably how successful Thor: Ragnarok is at the box office. If the movie does well, then it's likely that Marvel will want to make another Thor movie. However, if the movie doesn't do as well as expected, then it's possible that Marvel will decide not to make another Thor movie.
Another factor that could affect whether or not there is a Thor 5 is the availability of Chris Hemsworth. Hemsworth has said in the past that he plans on taking a break from acting after Avengers: Infinity War and its untitled sequel. If Hemsworth takes an extended break from acting, or decides to retire from acting altogether, then it's unlikely that there would be a fifth Thor movie without him.
Finally, it's worth noting that Marvel has plans for many more movies beyond Avengers: Infinity War and its untitled sequel. So even ifThor: Ragnarok is successful and Chris Hemsworth is available to star in another Thor movie, it's possible that Marvel may choose to focus on other projects instead of making another Thor film right away.