Ok, first things first. Go to [link=http://www.rcbatteryclinic.com]RC Battery Clinic[/link]. It's probably THE resource on the internet to learn about batteries that we use in our RC planes. It should clear up all of your questions.

But for a quick answer of your question. If your battery is reading 4.79 volts then you probably still have plenty of flying time. I borrowed the diagram below from the RC Battery Clinic for a quick explanation of this. Ni cad batteries do not have a linear discharge rate meaning that the battery discharges at a constant rate as you use it. Looking at the diagram you can see that the battery will discharge to a certain level (approximately 4.8v) and then plateau at that voltage for a period of time before continuing to drop voltage. How long will it stay at that level? That depends on usage, battery state, temperature, and about a dozen other factors. This is where a battery cycler comes in real handy for finding out how much time a battery has left in it. You can do a "poor man's cycler" by taking your plane into your living room, make sure you have a full charge on the battery. Start moving all of the servos like they would if you were flying and time how long it takes your battery to discharge. Check your voltage levels as you go along. This will give you an idea of how long you can safely fly, which should give you a little piece of mind. You should be able to fly safely until your batteries drop to a level of 4.4-4.6 volts. When it gets down to that level it's time to pack it up for the day.

Hope this helps

Ken