Yeah, I will 'change' my position on this. Satoshis in a single UTXO that are split are fungible - in your scenario you cannot identify the specific sat: either a1->a3, or a1->a4. This is kind of like 'forward fungibility'. You can however trace the value in satoshi's back from A4 to their coinbase at birth. At best, we introduce a UTXO taint percentage from previous inputs. Its this taint from other, external inputs that break backwards fungibility (due to taint).