Why would someone pay almost $20,000 for a Bruins jersey? Because it’s a No. 4 that was not worn by Bruins Hall of Famer Bobby Orr. The old-style wool jersey worn in 1957 by hard-hitting Bruins defenseman Bob Armstrong fetched $19,194.33, according to Lelands.com. Josh Evans, founder of the online auction house, says wool jerseys are hard to find these days, but the No. 4 on the back of the vintage Bruins sweater makes it especially valuable to collectors. Armstrong, a tough customer who dropped the gloves frequently during his playing days, skated for 11 seasons with the B’s. (There’s a spot of what appears to be blood on the jersey.) Three more Bruins players — Pat Stapleton, Bob McCord, and Albert Langlois — would wear No. 4 before Orr joined the team in 1967. The number was eventually retired.