"Raj will go to Delhi after four years."

Translation: राज चार साल बाद दिल्ली जायेगा।

[inbetweener]

Why baad and not ke baad.

[mayateacher]

In better English we say: Raj will go to Delhi in four years.

[Kanageshwa]

Why not ke baad

[vinay92]

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।