There is an important distinction to make between 4 channels and 4 decks.
Channels is the number of actual routes you have to play your music. Usually one look at a controller or mixer will tell you how many channels it has. Effectively there will be a channel strip for each channel, which will -on average- have the channel fader (with or without channel signal meter), cue button, EQ (and sometimes dedicated filter knob) and often a trim knob. Depending on the level of your controller/mixer there can be various other buttons and knobs, like channel input selector (something like CD/Phono/USB for example) and FX assign buttons.
Each channel means you can play the sound from that channel and control all those features individually for that channel. So, on a 4-channel controller you can have two TTs hooked up for example and your laptop and control all 4 decks with the four faders (channels).
If you have a 2-channel controller, they will often have the option of controlling 4 (software) decks. Most DJ software will allow you to have 2 or 4 decks to play with (some even more, but that’s beyond the scope of this answer). Those controller will have some way of switching between deck 1 or 3 on one channel and deck 2 and 4 on the other channel. While this may seem handy, you only have one fader, set of EQs and such to control two decks. In my experience this makes it rather hard to keep all the controls straight. If you think 4 decks are what you need, I always suggest getting a 4-channel controller. While you still will only have one pair of jogwheels, the margin of error gets a lot smaller if you have 4-channels for a 4-deck setup.
As for the question do you need 4 DECKS to mix properly, the answer -imho- is no. Many DJs that play wonderful sets with just two decks. The more technical and showcase you get, the more you might feel you need more decks. With the current state of affairs, a seperate controller for your Traktor Remix Decks/Serato Flip functionality, samples and such makes more sense in many cases. Here a 4-channel mixer comes in handy again, you can send the extra controller output to one of the “free” channels.
Hope that helps some.
.
