I had something similar happen last year - F4 where the taste which had been consistent suddenly is not. There was a distinct environmental effect - none of the plants in my big containers (the earliest looking seedlings) were the tastiest - all of the fruit in smaller pots were the taste winners, including 'extras' that I crowded together in one pot. But I cannot trust that it is entirely environmental, so I saved the tastiest to carry forward - but I also saved seed from the 'earliest' as a backup in case that trait was really lost.
The same thing happened with F3 of my Whiskeyjack, nothing really close to the delicious F2, and I had another F2 where the parents tasted very different, and they were all over the place for taste. I had another fantastic F2 another year, which I really didn't recover the taste in half dozen F3s. There were too many variables in play - a gf allele for one, some linked traits etc.
So my thought is that, setting aside environment, there may be a generation where taste traits that depended on heterozygous contributions are easily lost, and you have to grow more of that generation to recover or keep it. A half dozen plants might be enough for most, but in some cases you may need more. Maybe as many as 16 +.
Still it doesn't make sense to me, if the half dozen F3 were all tasty, that you don't find any good ones in F4. So that weighs on the environmental causes side.
If I were you I'd grow both F4 and F3 this year, so you can compare the two in the same conditions. If the F3 are all tasty again and the F4 are not, you know you have to save seeds from several of your best F3's and try those instead, I think.
