Lock a UE4 project?

No, if you want them to be able to load the project in the editor it can’t have something like that. That’s potentially a feature that Epic could add but they’d have to implement it into the editor/engine
Alternately, if you just want them to look at what you’ve made, you can create a build of your project so they wouldn’t be looking at your project files but instead just an output where the files aren’t editable. But that would only be for if you just want to allow them to look around at the map.