4 years is not great, but not unheard of either. There are many factors that effect the lifespan, including heat, vibration, initial shelf age, number of deep discharges, freeze ups...
Check the terminals first. Clean and lube as needed.
Check the water level in the cells. May be low after 4 years. Add distilled water as needed, and recharge fully, ideally with a battery charger.
Note: for a discharged battery, do not fill completely to the line, since the level will rise as it charges, and it may overflow.
For testing, a voltmeter is the way to go.
Car off, charged battery should produce at least 12.5 volts, preferably more.
At 2000rpm, no accessories on, you should get around 13.5volts.
At 2000rpm, with high beams on, rear defog on, blower fan on high, you should get a minimum of 12.5volts.
Best of luck.
1986 300E 5-Speed 240k mi.