Hi, 

I would like to compare cytokine levels of four groups of mice with different treatments. There are four to six mice in each treatment group, so the sample size is not large. The standard deviation of most cytokine levels in each treatment group are much larger than their mean.

Also, there are many zero in data set( very low cytokine levels).

What type of tests should I use to compare cytokine levels in different treatment groups?  

Any help is greatly appreciated.
Mann-whiteny test can be used as a non-parametric test to compare the median of two independent groups (samples do not follow normal distribution)