Battery not being charged, had a fully charged battery, took it out for a ride. died about 5 miles, pushed back. checked battery. had 4 volts

Voltage regulator may not be charging...gain access to battery and take reading with key off ..should read 12.6v or better...then turn on key and see if lights put on a load and voltage drops..then start engine and crank to 300rpm and with votmeter you should see it raise uo quickly to about 13.5 or 14v and immediate drop back down to about 13v ,,,if you got a decent battery,,,this will prove that the regulating system works. if not your stator is bad...find the connector to the stator and check any two wires leading to the stator for .1to1ohm this is a good reading, if not stator is bad. you said you had 4v is that correct? because if it is, your battery is shot and needs replacement.