Why baad and not ke baad.
In better English we say: Raj will go to Delhi in four years.
Why not ke baad
बाद is used here as an adverb. You can also use the postposition के बाद but then you'd need to use the oblique form of साल which is सालों.
राज चार सालों के बाद दिल्ली जायेगा।