Flyboy,
He is kinda right.
The pump has two ratings.
Flow, as in Cubic Feet per Minute. (CFM)
Pressure, as in Pounds per Sq. inch. (psi)
You need to be just as concerned with flow as pressure depending on how you want your bubbles to "look".
You need to push out all the water in the tube. The water depth is 4 feet so you need to push it down with around 2 psi (1.76) just to move the water. There is a pressure drop in the system that will need to be overcome with more pressure. This is because of friction and viscosity. There are tables that you can use to avoid all the Reynolds calculations and such. You will need to know the flow rate which means you will have to convert your CFM from the pump into GPM and then find the pressure loss. This also will depend on the material of the pipe.
Example, smooth pipe 2 gpm (min recommended) in 1/2-inch diameter pipe has a pressure drop of 57.2 feet per 100 foot of length.. Compare this to 8 feet of loss for a 3/4-inch diameter tube.
The pressure drop is less for large diameter tubes, so your friend is right. But, more volume of water in the larger pipe is harder to push out with air unless you have a large Flow rate (CFM). Otherwise, the air will bubble past the water or go over the top of the water and not push it all out...this might not be a problem. Larger CFM pumps also cost more. There is also a burst pressure rating on the tubing, but you are only talking about 10 psi or so.
I use 35psi and 25 CFM to blow the water out of my sprinkler system.
I'd be interested to know what you are doing, there are other things to consider.