You can't. CS4 is totally out of the upgrade loop in that sense.
You can either buy CS6 at full price (some $700) or go for any of the monthly subscription prices starting at $9.99 per month with a full year commitment.
If you have the 5D Mark III I'm assuming you are pretty serious about photography. In that case, as mentioned above, go for the monthly subscription--it's well worth it. Also you might still be able to get the photography plan which includes Lightroom CC and Adobe Photshop CC for the $9.99/month.
Check out this link below:
Hope that helps.