For me, there are a few factors that would help me decide:
Price - Note 3 has been out long enough now that you can get good deals on buying it. Contract? I'd avoid. As Madd54 said, tying yourself to 2yrs on an old model probably not a good idea unless price is the key issue.
Spec/Features - This depends on the rumours. If they are to be believed, the N4 will be an evolution rather than revolution compared to the N3. Once the N3 is upgraded to the same TW and Android versions as the new phones, will there be enough difference to justify it? For me, that would be no.
Patience - Are you patient enough to wait for the N4?

Availability - The N4 will probably sell like hot cakes and we'll see a lot of N3's coming onto the second hand market. If the N4 is only a step up from the N3 and snapped up by the early adopters, perhaps the N3 (with upgrades) will seem like a much better deal for a while?
So many things to consider. If you're patient, have lots of money free and have to have the latest and greatest? The N4 is probably a no brainer.
I've always had flagship phones the day of release on contracts. My N3 is the first phone I've ever bought outright and I'm now on a rolling 30 day sim only deal. I would never go back to being tied to 18mths or 2yrs just to get the newest. Waste of money imho.
Just my 2p worth
