My readings for nutrients per gram, per gallon of water are:
Epsom salts: 70ppm; .14EC
Ca/Mg (GH): 30ppm; .06EC (per ml)
Maxi: 110ppm; .22EC
So you can calculate from there what the readings end up looking like if you want.
Epsom salts are an easy application for magnesium. I use it because without it I start getting magnesium deficiencies, especially starting around 3rd week of flower. I generally go light on it (1g) before then, and ramp up as needed. If you're not finding signs a magnesium deficiency don't use it.
In terms of volume or timing of water/feed...it's going to be dependent on a bunch of other inputs in your grow.
That little auto I harvested was planted at the same time as the photoperiod...it was in the same sized bag (5g), but really got watered half as much in terms of frequency as well as me only giving it 2 gallons of water/feed rather than 3. I knew not to feed as often or much because: 1) the medium didn't dry out nearly as quickly though it was in the same environment as the photoperiod and 2) when I did feed it, I got enough runoff with 2 gallons rather than 3. Schedule, amount and strength are all dependent on what the plants tell you they want or need.
Also, warmer temperatures mean you'll feed/water more because of evaporation. Humidity of your environment also counts. The amount of light you provide for your plants is part of the equation, and of course each strain, pheno and sometimes just individual plants drink different amounts. Though after a bunch of grows with the same gear, timing, etc. I have a reasonable idea of what to expect.
I generally don't do 4 plants at a time, because the tent gets tight as well as I just don't need to maximize quantity. Also it takes more time to feed/water, tend to the plants themselves (train, snip here and there, etc.)...So I only do 2 at a time and I grow them out a bit more before flowering so they fill the tent. But yes, it would mean 12 gallons of feed each time if I had 4 similar plants going. It's not horrible though, because I'm putting the same amounts in across all the gallons then I'm pH testing then adjusting only one of the gallons and once I know how much to pH Up that gallon I just do the same to the others and don't test any of them.