I'm not sure what speaker size the DC4's run but assume it's 6" or 6.5". I'd personally go with JL Audio gear for speakers. Good quality sound.

Fronts: JL Audio C2600 6" 2-Way Component System - $228
http://soundmaster.com.au/JL-Audio-C...peakers/C2600/

Rears: JL Audio C2600X 6" Coaxial Speakers -$178
http://soundmaster.com.au/JL-Audio-C...eakers/C2600X/

4 Channel Amp: This Soundstream is powerful and seems to be good value for money: Soundstream TX4.560 4-Channel Class AB Tarantula Series Amplifier - $329
http://soundmaster.com.au/Soundstrea...lifier/TX4560/

In terms of sub, depends how much doof doof you want.. In my car, I just went with a 10" Kicker slimline sub to save boot space. It's not the most powerful sub, but it's great sound for the size and I need my boot space:
Kicker 10inch Ported Enclosure with 4 ohm CompS Subwoofer - $199
http://soundmaster.com.au/Kicker-10i...oofer/TCWS104/

Mono Amp: you need to make sure you match the sub and amp power output at their impedence level. For example, the above Kicker Sub is 300W at 4ohms, so I went with: Alpine MRX-M55 1100W Mono Block Amplifier which is 350W at 4ohms. I'm very happy with this amp and it gives enough power to the sub. - $234
http://www.ryda.com.au/Alpine-MRX-M5...r-p/mrxm55.htm

Installation Accessories:

You will also need at least a 4AWG Wiring kit which can be expensive. Kicker is a good quality one, but you can find cheaper brands out there
Kicker ZCK44 4 Channel 4 AWG AMP Kit 1000 Watts Max - $159
http://soundmaster.com.au/Kicker-ZCK...tts-Max/ZCK44/

Another set of RCA's for the Sub/Mono Block

Distribution Block to split the Power Cables

The above will cost you about $1300 - $1400, then you'll have to pay labour for install or do it yourself.

For car audio, I shop between Ryda or Soundmaster, but generally I just go to my local JB Hifi if they have the gear and get them to price match it but sometimes they don't stock all the products:
www.ryda.com.au
www.soundmaster.com.au

Hope that helps..