In order to install CS 6 upgrade, the previous version is not required to have installed on the machine. but you should have serial number for the same.
When you install CS 6 upgrade, it will first ask CS 6 serial , then it will ask CS 4 serial number.
I am not sure which application you purchased.
The CS 6 upgrade you are talking about is just the PS CS 6 or the full CS 6 suite ?
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Thank you Baljeet! This is my answer....
USE THE CS CREATIVE SUITE SERIAL NUMBER, NOT THE PHOTOSHOP-SPECIFIC SERIAL NUMBER.
I WON. It worked. Here's how:
I bought Photoshop CS6 online. I followed all the downloads correctly.
The icon appeared on my desktop. The information on the "Read Me" was only relevant for people how were installing from disks. (rrrggg)
When I got to the first install screen, it told me it did not recognize any related software on my computer (why the heck NOT?!?!)
But I started to complete the form, as asked.
I forgot that I had Photoshop CS4 Extended. I inadvertently had the non-Extended version slelected, which it would not recognize.
BUT ( ! ), I had gone to my Adobe products and wrote down my Photoshop CS4 Extended Serial Number from the site.
I re-attempted the install using the CS4 Extended product selection and my current Photoshop Serial Number. Still didn't work.
BUT. But, I then entered my CS4 Extended Creative Suite (Design Premium) Serial Number, and it took that.
Why it wouldn't recognize that Photoshop was already on my machine in the first place is Bad Adobe.
As you mentioned, you purchased the upgrade from Creative Suite CS 4, not from Photoshop, So CS 6 will only recognize the serial for Creative Suite not just for PS CS 4 extended.
Even if you not have anything installed on your machine, you can still install CS 6 upgrade and then choose the older version product you have and enter its serial number.
This is how Adobe upgrades work.
This mechanism gives you the liberty to install upgraded versions without even having older version installed on the machine. You just need to have the serial keys.