Can engine run with only 2 out of 4 spark plugs?

[lncovert]

My engine only has 2 out of 4 spark plugs. It was like this when I bought it apparently, and it has been running great for the past year. Is there a reason why someone would've taken out 2 of the 4 spark plugs? Thanks for your help...I am very new to engine troubleshooting.