Need some serious advice here.

I have a gap of 1 year from my degree completion year to joining my 1st company and a 5 months gap between my 1st and 2nd company. I worked for 2.5 years in my 1st company and 10 years in my 2nd, 3rd and 4th companies. My 3rd and 4th companies never asked me for the experience letter of my 1st company. Considering this I showed my total experience as 4 years (covering that 1 year and 5 months gap) to my latest company (5th one). But they have requested for the experience letter now. What shall I do now?

1. Change the dates in experience letter.
2. Come up with some excuse saying 1st company was acquired by some new company so mistake in dates.
3. Tell the truth
4. 1st company is not responding Please suggest.