Bound does not work like that in V4, but regardless you cannot observe it because it does not include the normalization term.
There is a draft PR opened recently: Implement Truncated distributions by ricardoV94 · Pull Request #6113 · pymc-devs/pymc · GitHub
In the meantime you can implement a truncated likelihood by adding a Potential term with the normalization constant.There is a v3 example here: Google Colab which came from Help with pm.Potential - #12 by sspickle