On the Vi-tal you can use it for 1-2 days( before & after transportation ). 3-5 days ( stress related needs or to stimulate appetite during tempeture extremes) or 10 days(for aid in nutritional build up when feed intake is below normal). There is a high end and low end mixing guide on the packet. Then there is a small one inch square on the packet that has the mixing guide for small amounts on it. I almost never found that part on my packet. Look at your packet really good and see if it doesn.t have a little amounts guide on it also. If it doesn't maybe you could call the 1-800 number on the pack and the could help you out.
You just have to do the math. Weigh out 4 oz and see how many tablespoons that is. If you don't have a scale, you can guesstimate by comparing it to a something of a known weight (perhaps a quarter pound package of cheese, for example). Or you can look at the total weight of powder on the package of electrolytes. So, if it is an 8 oz package, 4 oz is half that. Then measure the volume of that 4 oz of powder. How many cups is it? (4 oz of weight does not equal 4 fluid oz) You'll need some conversion factors (such as number of TBSP in a cup, number of tsp in a TBSP) which you can find on the internet. You get the idea.
Then figure out what volume of water you want to make up. One gallon, perhaps? Then, the amount of powder you want would be 1 259th of that 4 oz measure of powder. It might come out to something like a quarter teaspoon.
The fact that a chicken is smaller than a pig does not necessarily mean you need less of that powder per volume of water. What you're looking at is the concentration of electrolytes in a given volume of water. It makes sense that most animals would use a similar concentration of electrolytes. After all, we are all made of cells containing similar concentrations of electrolytes, and blood serum of similar composition.
But if you are nervous about using the same concentration for a chicken as would be used for a pig, go ahead and use half the calculated amount. It won't hurt to use too little. After all, many people use none.