sram, you are saying that you want just four varibles (you might be meaning 'four factors') out of six from the data of 24000 subjects or what? PCA gives out important components or factors, not the individual variables. So,for example, if you identified 2 important factors from a possible combinations of 6 variables, these two factors can consist of 2 or more variables. It is not necessarily 4 varibles you want. Might be I understood the problem wrongly.