First, that '4 watt portable', if it's one of the 'bubblepack' radios, is probably actually 1.5-2 watts ERP.
Second, once you reach 1-2 watts, power is essentially meaningless for simplex communications.
Antenna height and terrain is everything.
In completely open terrain, with antenna heights (handheld) about 5 feet, say, over seawater, your range MIGHT reach 4-6 miles. In urban areas, you'll be lucky to get 1/2-1 mile. Downtown center, consider yourself lucky to get 1/4 mile.
Before buying radios, you should find out if there are any repeaters in your area, and whether you can get permission to use them if there are. If you can, you need to find out what sort of radio you need to access them. Some do not use the 'common' CTCSS tones, some use multiple tones (different transmit and receive tones), etc. You cannot use a repeater without permission of the owner.