Offer letter for a newly registered company operating from the past 4years

I have been working in a company from the past 4years but the company got registered as Pvt. Ltd. recently in 2017, in that case what will the joining date of mine and do I need to furnish two different offer letter of the company for date of joining to other employers or one offer letter will be enough to explain my employment with this current company for 4 years to other employers.?...please help.

Dear Rakesh,
Let me be clear, is't like joined in an unregistered co. and later on this was regd. and continued it's business/with same employees as in the past ?
Change in status of the employer does not impact in change in the joining time of its employees. Ofcourse in instances of employees continued but change in the names of entity (unregistered to regd.) it might. This can be overcome by issuing a formal letter to everyone stating the continuity of service is validated without any break. This formality would ensure full entitlement in respect of EPF/EPS, Gratuity, bonus, confirmation, increments & promotions. However it's inevitable/essential to appropriately arrange name change, if any, in statutory documentation in registrations such as EPFO, ESI, PT, PAN/TAN/TIN, GST/ Labour licences, MSME etc.