Only the slot is upgradeable.

No, you cannot upgrade the soldered RAM without replacing the whole motherboard. The slot stays perfectly usable, but the 4GB soldered on is permanent.

This is the whole trade-off with hybrid RAM laptops. You get the benefit of a slot for future upgrades, but the soldered portion is locked in at the factory. If you add a 16GB stick, your total becomes 20GB (4 + 16). You can’t remove that 4GB or swap it for a larger chip.

Could someone with a hot air rework station desolder the RAM and replace it? Technically yes, but it’s risky and expensive. Most repair shops won’t touch it, and you’d still need matching firmware support. Not a practical upgrade path for normal people.

If you’re buying a laptop with this setup, just accept the soldered amount as the floor. Plan for the slot to be your real headroom.