Hi there.
I have joined this forum specifically to post the results that I have experienced after reading this post - I'm a firm believer in spreading the love. If this helps others in the same way that it helped me, the world will be a better place.
So...
I was actually using my trusty yet genuine version of Reason 4. I recently bought a Macbook Pro running OSX 10.9 - and, like others, I found that I was getting the message "Can not open Reason Engine, Please re install to make sure reason engine is properly installed."
After considering the option of downgrading the OS, or having to spend more money on more software (thus rendering my copy of Reason 4 mostly useless) I decided that Google would have to be my best friend, and found this forum. I followed the instructions exactly and it worked a treat...
1) Download the newst demo version of Reason from the Propellerheads site
2) Install and run the demo in 'demo' mode
3) Close the demo version
4) Locate the 'Reason' folder in 'Applications' and rename it (I changed it to 'music software'
5) Run the Reason demo in 'demo' mode
6) Install Reason 4 as normal
7) Close both versions of Reason
8) Run Reason 4 as normal
It works a treat.
Hats off to DP user 'Menace'. Good work sir.
Simon, out.