adding water to fermentor?

I need some advice. I am brewing with very limited supplies. I have a 5 Gal pail with bung and airlock. At home I use a 30 lit (6.5 Gal?)pail.
I am concerned that the krausen will foam over in such a small pail. My question is can I make up my extract kit with only 4 Gals of water and then after 2 or 3 days in the fermentor add another Gallon of water? Thanks in advance for the help.

Yes, you can. Just make sure you've boiled and cooled the water so that it is sterile when going into the wort.

Also, if you have tubing, it will probably fit into the hole for the airlock and you could use it as a blow-off tube. Blow-off tubes are good when you know you'll have a very vigorous fermentation or when you have little headspace in the fermenter.

Whether you need a blow off tube depends on what type of yeast you are doing and at what temp you are fermenting at. Being in Nova Scotia I don't believe you'll have too much of a heat problem (especially at this time of year). I make it a point to ask my LHBS how much activity to expect out of the yeast I am buying from them. I.e., Does the yeast ferment aggressively, or does it have a tendency to to be a little slower?
Writing down everything that happens to your beer in a log/journal will help you keep track of what beers/yeasts require blow off tubes.

Once my beer is fermenting, I don't like to mess with it and for most beers your pail would be fine.

If you think you might have blow off and can't rig a tube, you could always just put the pail in a bathtub or large container and just rest the lid on the top without sealing it. This would allow any excess krausen out safely without having to mess around adding water later and chance watering down your beer.

Thanks for all the info guys. I am in west Africa where there are no LHBS and am trying to make a batch with some non typical supplies. I have 20 litres in my pail now and will boil, cool, and add 3 more litres in a few days. The next adventure will be trying to bottle it!