With the HS4 it is a debatable about a failsafe. The repay will protect against a pulled loose probe. The HS will also shut down if a probe wire or probe breaks. The only point where a failsafe would be needed is a full catastrophic T-stat failure. It can happen not overly likely. The use of a good surge protector would add a bit more safety to the HS4 as surges can cause issues with the Triac controllers.

In the end only you can decide what is reasonable or not. Personally the new Herpstats are the only t-stats I would ever even think a fail safe was not needed. I own a herpstat 4 and I have a fail safe used with it. I don't know if that helps, I felt that the cost was worth it. My decision, you will have to do what you think is right for you and your animals.