Lots of experience

1. I gained lots of experience is most commonly used. experiences means different kind of experiences but it is not commonly used.

2. 4 year working experience is correct. in this case, it is incorrect using experiences.

Note. it is not necessary to add "-" between year and working. If you say 4-year means a specific project or school with a 4 year term.

Exaple of using experience in plural form - I have a lot of experiences at work. I was the buyer in the purchasing department for 1 year, accontant for 1 year, production supervisor for 1 year and administrator for 1 year.

Hope this helps.

Francis from Canada