You can. And you can keep both CS4 and CS6 on your computer at the same time.
There is no upgrade pricing from CS4 to CS6. You'll have to pay full
price. If you only want InDesign as a standalone application it's going
to cost you $700. The suites go upward of $1800. You may want to
consider Creative Cloud.
Thanks, Bob. I was afraid I was going to have to pay a lot. I don't want to do the CC option, so will just have to bite the bullet. Have a great holiday season!