Not necessarily the MB. You need to know the BIOS mfg and Dell model could help too. Open it up and look for the BIOS chip to see the mfg.
Then with that information, look here: »kb.iu.edu/data/afzy.html
or »uk.answers.yahoo.com/question/in ··· 8AAEB9CX
or do other web searches using "computer 4 beeps at startup"