生日優惠即是四位同行
birthday special is for four people traveling together.

Cantonese Sentence Breakdown

生日 sang1 jat6
birthday
優惠 jau1 wai6
specials; discounts
即是 zik1 si6
that is
四 sei3
(slang) labourer
位 wai6
position
location
place
seat
classifier for people (honorific)
classifier for binary bits (e.g. 十六位 16-bit or 2 bytes)
throne
post
rank
status
同行 tung4 hang4
travel or go together