I don't intend of having the same pair of boots for 30 years...... 4 years of good hard use is great imo - a 30 yr old pair of boots for sure doesn't have the same amount of cushioning, support, insulation, and physical structure as a newer pair, so toss em would what I would say to you.
The newer boots these days are like wearing air on your feet, with better warmth and waterproof protection than ever before. The down side is the durability of these materials isn't like your wearing a animal Hyde boot that weights 15 lbs.
I'd take a newer tech boot any day over an old style cement bock boots of yesteryear.