It depends on your recipe. For instance, if you want to make a 3lb recipe and you have a recipe like this (this is just off the top of my head so I have no idea what the numbers would be or the properties the bar will have. It is just for illustrative purposes):
40% olive oil
15% coconut oil
15 canola oil
15% palm oil
15% palm kernel oil
You would figure your recipe this way:
3lbs converted to ounces is 48 oz. You then take the 48 oz and multiply each percentage amount, changing the percentage to a decimal, with the 48 oz.
40% olive oil would be .40 x 48 = 19.2
15% coconut oil .15 x 48= 7.2
15 canola oil .15x 48= 7.2
15% palm oil .15x.48= 7.2
15% palm kernel oil .15x48= 7.2
Total 64 oz.
You would do the same calculations for any size recipe. Just convert the weight of the recipe to oz and multiple, change the percentages to decimals and multiply the decimal amounts with the total weight of the oils and butters. I hope that made sense.
Now if your mold holds 4lbs, you will also have to account for the lye/water solution. But this is how you can figure out and resize any recipe.