I need to find the earliest date out of four fields but they are not next to each other.

I have seen the below formula:

=MIN(IF(A1:A4>=01/07/15, A1:A4))

However I can't seem to make it work when not selecting a list to choose from i.e. I want to choose from WT2, ABO2, ABR2 and ACP2.

Can anyone advise what formula I need to use?

Thanks so much!