I too purchased a bunch of Mitsumi discs from FutureShop and have since determined that they are very poor quality discs.
Use DVD Identifier to determine the real manufacturer of the discs.
Chances are that the 4-pack and the spindle are made by different manufacturers - even though they both say Mitsumi.
Go with a better name brand disc and chances are that your problems will vanish. The 1620 prefers +R discs so make sure they're +R's as well. Read some of the posts in here to determine what discs people like for their 1620's.