Significant differences in significant differances

Hello forum Gurus,
I have tried search for a problem similar to mine in these forum to no avail. So here goes. I have 4 groups (A,B,C,D) with 4 biological replicates each. I have calculated the fold change in gene expression between Group A and Group B as well as between Group C and D and if that fold change ratio is significant or not. Now I want to determine if there is a significant difference in the A/B ratio and the C/D ratio and I am stuck on how to do this. Any suggestions are welcome and thanks ahead of time.
I am thinking that where the data came from isn't relevant as much as how I would like to analyze it. Most of the time the expression data is normally distributed if that helps with suggestions