Been pretty strict when I'm dieting, I think I posted it somewhere but I'll do it again. You can actually lose much more than 6kgs in 5 weeks depending on your start weight, and also remember I was overseas for part of it so couldn't keep up with the diet there.
Pretty much follows the ideals of PMSF, which is having enough Protein to keep your lean muscle mass (otherwise you'll just end up being skinny fat which IMO is worse than being obese). PMSF usually involves a huuuge calorie restriction. For someone who has 60kgs of lean mass it'd mean having like 150 grams of protein a day and not much else which is like 600 calories. I'm roughly 65-70kgs of lean mass (~18 kgs of fat) so I just eat 250 grams of protein (a tad over the minimum) which is 1000 calories, and then another 400 calories of other things for nutrition. That's 1.4kcals a day input, and my maintenance sitting around doing absolutely nothing is 2.4 kcals.
That's 1000 deficit from food a day, and then the rest is from cardio/gymwork.
1500 calorie deficit a day is about equivalent to 1.5Kgs a week and then yeah 6kgs in 4 weeks. I know you three have me on fb, just check pics from around March-June, and then something more recent in India and you might notice a fair difference. (It's actually like 20kgs, though I personally can't notice it as I see myself everyday =\)
But yeah, you can lose 6kgs in 4 weeks using what Matt did as well, I'd actually encourage you use that method if you want to cut only 1.5kgs a week. I'm going for something a bit quicker. You're also spot on Matt, knowledge is everything.
A general safety warning, do not try the strict PMSF unless you know what you're doing, it's very dangerous if you don't do/eat/take the right things whilst doing it. It's generally used by bodybuilders and the morbidly obese to shred several kilograms of fat within a short amount of time, and you can't keep it going for long.