How write in CV about lecturing experience while being a PhD student I am currently PhD student, and giving lectures to undergrad students (4 semesters). Is it legit to write about this activity in the CV separately. If yes, then how (i.e. lecturer)?
If lecturer is an actual title in your location, as it is in many, and you don't actually hold the title, then it would be a mistake to suggest otherwise. If you have full responsibility for a course then you could provide a list of "courses taught". But if you are just a guest lecturer in a course taught by another, you could provide a list of "presentations" with a bit of detail. 

Don't say things that others might consider misleading. 

You could also seek advice from your current department/school as to how to properly present yourself. Likewise, a professor that you worked for in such courses can provide some guidance.