Can I sue a company for an idea I mailed to them 4 years ago that they are developing today?

If you can prove that they are developing the product based upon your specific and detailed idea not just a general concept and they signed a non-disclosure agreement before you sent them the idea, you can sue for unauthorized disclosure of proprietary property.

Its possible if you have an issued patent.

You can try - but it is likely that no attorney will take your case. Unsolicited disclosures are usually either returned unopened or they are thrown away - company policies differ. Most companies have these policies because they have numerous ideas in development which can take many years before a product is launched. You likely approached the company because your idea was something that you believed would fit in their current product line. Unless you already have a patent on your idea before you send it to them - you have very little chance of success against them - especially four years later.