Thanks for posting the available fund choices and their returns. We also need to know what the expense ratios are (ER), or the charges to use the fund. Also, is there a fee for the DC plan?
The 4% dividend is quite nice. Do a little research on the company and see how the dividend has been over the past and see how the stock price has been too. Check to see if your company is one that has consistently improved the dividend; some companies are known for that.
The diversification is important-Enron. If you were to stay with the company for a long time, just holding its stock could be scary, mediocre, or awesome. Nobody knows, but that is why a shrewd investor spreads the risk in a broad index of different asset classes that move somewhat or a lot differently on economic conditions, e.g., stocks (large, mid, small, value), bonds, REITS, international stocks.
Have you read the Bogleheads' Guide to Investing, William Bernstein The Investor's Manifesto or any John Bogle books? They will help fill you in on some important investing ideas.