Copy Cell Data from one sheet to another without overwriting

Hi
I am still new to excel VBA and my biggest problem is that I don't use it often enough. I have used this forum to get answers to quite a few questions, and I have had help in the past, but this one has me stumped. I actually have four identical workbooks that are generated daily, but I usually copy and paste the data into one workbook with several sheets. combine them manually, then use this data from the first sheet to make the workbooks for the next day. This is done so we don't read data from areas that have already been read. I've attached a sample. We then go, take more readings each day, I combine them. We do this each day until all the rows are filled. The first two columns are for reference only and the data is filled in the third and fourth columns I need a way to copy from sheets 2-4 into sheet one but not overwrite anything, or at least have a message box pop up to ask me if I want to overwrite the cell.
Thanks for any help.

GW