Beta for v4 has a tenative date of the end of July. Stable release date is a bit fluid but sometime in August.
My opinion on upgrading will have a bit of personal and company bias, with that said yes you should upgrade eventually but we have no idea if/when MongoDB/Mongoose will release their connector for the v4 and all v3 support will cut off (from all parties) 6 months after the release of the v4.