I think the 5th one is sorta hidden away, it's the phase inverter I do believe.
What are your settings? Try these for low volume jammin:
Lead gain: 8-9 pulled
Treble: 7.5. More if you want more gain but careful, it will get hairy quickly.
Mids: At low volume you could crank them to 10, or leave them high, around 7.
Bass: 0-2. I usually leave it at 1.
Lead Drive: 8. Pull it if you want but you will have to back off the gain too if you do this otherwise you will probably feedback.
Volume: 1
Output Volume: 3-5
Presence: At low volumes, I would pull it and leave it around 4.5-6.
This is on full power, pentode, class A. If you go to triode you can probably push the volume a bit more.
As for the EQ, at this low of a volume, you can crank the 80hz slider to max and get a nice meaty low end. The next slider is broad, be careful. There is a very, very fine line between **** and good. I find that when the 80hz slider is cranked, the 200hz slider is very touchy. I usually have it about 1cm above the center line, but play with it. Even like 0.5mm of change will alter your tone drastically. The 750hz slider I put about half way to the bottom. Again, this one is touchy, and this is the knob that will make or break the definition of your guitar. For Metallica tones, scoop it enough to bring the appearance of hugeness, but not so much that your guitar loses clarity. Again, play with it. The next slider I usually put about 0.5cm above the middle line or so, and the 6600hz slider I will put anywhere from just above the middle line to about half way up. Careful with this one too, it can make your tone sound very brittle if you aren't careful.
Also, keep it mind your guitars, pups, etc. Metallica uses EMGs which are super-hot pickups. A lot of the tightness and compression comes from that aspect.
Hope it helps.
But seriously, if you want Metallica, know what you gotta do? Lead volume & Master Volume = 10 on full power, pentode and simul-class. Then play.