I would consider splitting this up into two different fields to allow for the properties type to properly doing its thing. Have one field for the birthyear and another for the actual birthdate. This would also make calculations related to this fields a little easier as you could add in the needed part when you want to, instead of having to separate it out in the queries.
By using quotes around the date, you could also force it not to be a date. If not quotes in themselves are not enough, you could also add some other character (or emoji) forcing it to become a text.
Depending on what you use to query, it’ll still be able to transform back into dates. In dataview you could use substrings and the date function. But it would be cleaner to use two fields.