Do you mean InDesign 4.0 (CS2) and InDesign 6.0 (CS4) or CS4 and CS6 (which is version 8). The numbering system is a bit confusing since the introduction of the Suite packages.
You can no longer purchase a CS2 to CS4 upgrade through normal channels, and you should be very cautions about buing on the secondary market. Many copies are counterfeit or have already been used and cannot be transferred with also transferring the rest of the upgrade chain so you will not be able to register and activate.
A CS6 upgrade package should be available from any authorized reseller or direct from Adobe online. Cost will depend on which upgrade you need. If you have a full suite package you cannot upgrade individual componenents and mut upgrade the entire suite.
Hello Peter, it's from CS3 tup to the newest Indd-version.
Thanks for helping.
CS3 is the earliest version from which you can still obtain upgrade pricing. You might also want to look at Creative Cloud.