First soda making

I bought the birch rootbeer extract it came in 4 oz. It says it only makes 4 gallons but I have read that people have been making 5 gallons out of 4 oz. instead. any suggestions?

Also I have a kegerator and would force carbonate. At what pressure should I force carbonate it? And if I can only do 4 gallons would that be alright to put in a 5 gallon corney keg and still force carbonate?

I also hear that I should have a long about 20 foot supply hose. However I do not have that and maybe have about 6 foot hooked up to that tap. I do not want to have to take the tap back off and try to re hook it because it was a big enough pain in the first place to install.

Any help is greatly appreciated!

Check out the prior threads. They'll reference the second two.
1) Depends on how strong you want the drink. I suggest sampling in 2 liter bottles first to find the ratio you really want for several gallons.

2) About 30psi. (carbonation may vary)
3) https://www.homebrewtalk.com/f35/cure-your-short-hose-troubles-100151/