re: Company forgot to offer me 401k when I qualified now they're giving me $ explain

Ask the company's HR department for a copy of the plan document or the summary plan description (SPD) for the company's 401k and see if it has a profit-share or a match component. A profit-share (or match if you contributed money to the 401k) probably is what they are adding money for. Most companies have a vesting or eligibility period before you can receive a profit-share or match; however, I'm pretty sure that the longest permitted vesting or eligibility is less than 4.5 years. So, they could have not been following the terms of the plan which could violate the Internal Revenue Code or, as another poster mentioned, ERISA.

ETA: You don't have to enroll in the 401k plan to receive a profit-share.