I originally purchased Lightroom 4 on my Macbook Air which I recently sold and purchased a new Macbook Pro. I did not reactive anything on the old computer but the hard drive was wiped clean meaning Lightroom was deleted from the old computer.
To install on the new computer do I just re-download and and insert the key? Will that use up 1 computer use or will that be my 2nd?
It will be your first use. It was removed from the first computer during the wipe.
There is no activation/deactivation for Lightroom. The software doesn't count how many times you install it or how many times you use the serial number.
The limit that exists is that you may only use the same serial number on two or your computers simultaneously, under the restriction that you have Lightroom in use on only one of the computers at any one time.