Firstly, CS4 is a very old product and I don't think you can upgrade to any CS5 or CS5.5 now.
Currently we have the CC version for which Creative Cloud pricing and membership plans | Adobe Creative Cloud are available.
Secondly, for upgrading there is always a cost involved which is less than the full product cost.
Maybe I wasn't very clear in my question. By "upgrading" I meant "buying" the latest CC version of InDesign. I put buying in quotes since it is more like leasing if I have to keep paying for it as long as I use it. It seems that the CC version will quickly (within two years) become way more expensive than the old InDesign CS4 which I (for all practical purposes) bought quite a few years ago... And I tried the new version a couple of months ago but didn't notice anything new, making it easier to create stuff...
Well the CC version is a subscription based model only and requires either a monthly plan or annual plan.