It sounds like you've pulled very little of the potential flavor and alcohol out of your wash, so I'm not surprised the backset still tastes ok. It's not something I've ever done, so I have no experience with it. Your backset will be sterile as long as you haven't contaminated it since, so it might keep quite well in a fridge or freezer.
4% is an ok, beer/cider/RTD type, abv for drinks by the pool & BBQ, but you might not want to let your friends taste your backset unless it's April the 1st, or they are pests
Mahalo (thank you) for all the comments. Saved about 2.5 liters of the backset in the reefer. Yea it's like a weak beer, and a little sour. But I like it. Dumped the rest in the yard and cleaned all the equipment. Maybe I can get back to brewing beer which I "know" something about. LOL
Hey man just brew a beer, then instead of boiling and adding hops off the lauter just cool it and ferment it and you've got some winning single malt right there.
"A little learning is a dang'rous thing; Drink deep, or taste not the Pierian spring: There shallow draughts intoxicate the brain, And drinking largely sobers us again." - Alexander Pope