Your best option is to get a subscription to InDesign CC. It will be updated regularly for whatever operating system you're running, and will add new features.
Your second best option is to buy InDesign CS6 at full price. But you're buying a version that is 3-1/2 years old and may not work with future versions. It will never be updated.
You cannot buy InDesign CS4 from Adobe. If you find CS4 online there's a high likelihood that it is pirated and may not work.
In addition to Steve's comments, if you want to get into EPUB CS4 would be worthless. Even CS6 is fairly archaic when it comes to EPUB.
CC is the only way to go for what you want to do.
Edit: IF you're on Mac you'll need 10.9 at a minimum.
hmmph. I'm at 10.8.5 … so close. I guess I'll have to upgrade my OS. Do you have a recommendation as to what I should go to, or what I am limited to going to from 10.8.5? … I have heard there are bugs with certain ones… plus, would I still be able to use my CS3 software? (It been awhile since I've upgraded, I don't recall. Does upgrading one's operating system need a complete reinstall of programs? I think not, but could programs not work in a higher version… or are they not affected by it?) Thank you!!!
I will look into CC. I haven't heard much about it.
I have found in place upgrades of Mac operating systems to be very unreliable especially when it comes to Adobe apps. CS3 isn't even supported under 10.8 so you're nearing a point where you'll have to upgrade anyway.