[BioC] Comparision of expression values from 4 different studies

[Srinivas Iyyer]

Dear group, I have 4 microarray studies.  I am
interested in comparing the expression (expression
values) of my genes across these studies. 

I want to take expession value of gene A from first
study and and take expression values from other
studies, and compare them. Is this process
statistically allowed. or do i have to do some
treatments for all 4 studes, to bring to a some level
so that the expression values can be comparable. Is
meta-analysis is the way to go? please suggest.. 
thank you very much
srini