I did a blog post on this exact topic. http://www.analysisfactor.com/statchat/?p=35.
You don't need to impute the mean of the other four items for a missing value if you're using it as part of a mean. The mean of the four items with one missing will be the same as five items in which one of them has an imputed mean.
The mean of (1,2,4,5) is 3.
The mean of (1,2,3,4,5) is also 3.
And you really only want to do mean imputation if your % missing is small. Say, less than 5%. More than that and you'll start to bias your results. I did a free webinar on missing data recently. You can get the recording at: http://www.analysisfactor.com/learni...training6.html.