Gratuity eligibility is 5 year or 4 years 240 days ?

You should be eligible.

First of all, preferably, if suitable to you;

Submit notice of resignation, final resignation under proper acknowledgment and download POD also, and get acceptance of both,obtain NOC/NDC, correct FnF statement, FnF dues, service certificate, relieving letter with good comments,Form16, salary slip of all months, PF passbook /a/c slips of all years,ESIC card, acknowledgment of handover of charge etc etc ………………..and clear BGV ASAP

There after you may submit FormI under proper acknowledgment and mention that ‘Notice of Determiantion of Gratuity’ and payment has not been supplied despite representations in office/in person/by email/phone calls etc to Mr/Ms………………on dated………….. .

If Gratuity is not paid within 30days from LWD, employer is liable to tender  interest @10% for the delayed period.

There are many threads on similar query at LCI:

https://www.lawyersclubindia.com/forum/display.asp?cat_id=9&forum_id=49

e.g;

https://www.lawyersclubindia.com/forum/Clarity-between-gratuity-eligibility-service-5-or-4-8-yrs--28768.asp

You may pick up relevant points, download FormI, citations andtry to resolve with your own skills or lean on your employee’s/trade unions or your own counsel specializing in labor/service matters at your own location.

You are welcome.

I am giving below some Supreme Court cases references, which clarify your doubts.-

AIR 1984 SCC(4) 356, SC 1842 Sc(2) 303.

RF 1986 SC 458(3), RF 1988 SC 587(12). 

Ref.para.3.. Only Last drawn basic+DA (excluding all allowances ) should be considered for arriving 15days/year.

Formula - Last drawn basic wagesx15daysxno. of completed years ( divided by)/26.

You may search via internet for SC cases.