Just put "PN4889" at www.google.com - it came up with dozens of hits for electronic components.
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The link I copied above said Fairchild Semi for the PN4889 - I don't know if it's their chip or if the PN4889 is a generic designation. I don't have an ST140 anymore to pop open for a better look/guess. As for the Mosfets, MOST (but not all) ST140s had Hitachi output devices -- check out this archived thread for some more info:
If yours is black faceplate + champagne/gold rack handles, I gather it's probably got Hitachis. If it has the red/blue "sonata series" lettering on the front, it may have something else. Audiogon member "Sean" is the most likely person to know the answer to this.