Dandruff,
Welcome to the equipped forums! Being here is definately a smart way to start your preparations.
You are right that 1 US gallon = 3.785 liters (let's round up to 4 L to make things easier). Singapore's temperature stays pretty steady between 24C and 32C all year long and monthly rainfalls of 10-25cm and a daily average humidity of 80-90%. So, fairly hot and wet all the time.
Under those conditions 4 L per day per HUMAN does sound like a bare minimum. Now, not all this water is for drinking. It also includes water used to cook food, wash yourself, washing your dishes and even occasionally washing your clothes.
Your weather is similar to Houston, Texas where I live. We sweat a lot here. When I'm working outside I've drunk more than 4 liters in one day. Plus, being all sweaty I need a lot of water to clean up myself and my clothes (one can put that off only so long). Let's say I need 6L (more likely 8L) to stay hydrated and clean. I still haven't included water for cooking or doing dishes. Add another 2L for that.
Now, I guessing in your situation not all six people will be working hard, sweating, etc... Let's say only two of your are and the rest are just sitting in a shady spot trying to keep cool. This suggests you'd need at least 8*2+4*4=32L per day for the humans or 96L for 72 hours.
Four of
these will give you that plus a little extra. They are 35.56cm by 38cm by 28cm and hold about 26L each. That's not really much space. As Haertig mentioned, add the water from your koi pond (treated first) for cleaning and those water containers will last you even longer.
As for the pets, I don't know what sort of dogs you have but I do have two cats. I would say combined they use about 4L each per week, though probably a bit less. As for the dogs, measure how much water they drink to work it into your plans.
Now, on to food. Are there certian canned or boxed foods that you already enjoy, eat on a regular basis, don't require refrigeration and are easy to prepare? Things like pasta, dry beans, canned stews or soups, cereal, peanut butter, raisins, etc? Buy more of that. The trick with "survival foods" is to stock what you eat. To get extra calories add foods such as honey, peanut butter or plain sugar. Don't forget seasonings to spice things up. Make sure you do have a mix of carbs, fruits-veggies and protein. Also, if you are sweating you'll need to take in more salt so have a container of that on hand. Carbs in the form of oatmeal are a favorite in our household. With the right seasonings added you can make oatmeal taste like anything (I like it mixed with either peanut butter or barbecue sauce).
An easy way to build a stockpile of food is just buy a few extra cans/boxes/packages each time you get groceries. It'd be nice to get it all at once, but that's beyond the means of most of us. Oh yeah, it doesn't hurt to throw in a bottle of multi-vitamins.
-Blast
p.s. Do you have any good way of catching rainwater off your roof?