Want the positive value, ignore 0, negative, N/A ect

Hi All,

I have some data that updates every few minutes or so with no set schedule. The data could also update 50 times, or more, in one minute. Basically, I am getting the data from an external source so I have no control over it.

When the data is updating it can become "" or negative or N/A or anything else.  All I am interested in are positive values and want to ignore all the rest until the data updates again. As soon as the data updates again I want to capture it.

Therefore I would like to come up with a work around.

Here is what I am imagining.

Premise:
Suppose that I have a positive number in excel worksheet1 S4.  

I want to have a way to do the following:

1. As soon as the positive number in the Premise changes (up or down) I would like to copy S4 and paste it in to T4.

2. If the data is updating and S4 becomes zero, negative, nothing, "", or N/A or (anything but a positive number) I would like T4 to remain the same as it was before.  Once S4 is positive again then S4 would be copied into T4 immediately.

Thanks for your help!