Sooner or later immigration will probably stop you. There’s no set number of times you can do this but if they think you are abusing the visa exemption they will refuse entry.
If you want to stay longer then look at the other visa options, DTV for instance.
I would try entering visa free with the 60 day visa exemption twice then get a multiple entry tourist visa for the other two trips. This is just to be safe as they are getting more strict on this now.