a motherboard which has 2 pcie2 x16 ports,
Dimensões físicas
but has (x16, x4) written next to it.
"Pistas" disponíveis (conectadas eletricamente)
Am I right to assume that mean that only 1 port is true x16 and the other port is x4?
Sim e não,
If so, does that mean I can't plus in 2 x16 graphics cards?
Você pode conectar dois cartões x16 e eles funcionarão, mas um deles só poderá usar 4 pistas. (Acredita-se que seja possível que a máx. ultrapasse 16 e se 4 já estiverem em uso, apenas 12 estarão disponíveis para a outra placa)
A fonte de todo o conhecimento diz:
A PCIe card fits into a slot of its physical size or larger (maximum ×16), but may not fit into a smaller PCIe slot (×16 in a ×8 slot). Some slots use open-ended sockets to permit physically longer cards and negotiates the best available electrical connection. The number of lanes actually connected to a slot may also be less than the number supported by the physical slot size.
An example is a ×8 slot that actually only runs at ×1. These slots allow any ×1, ×2, ×4 or ×8 card, though only running at ×1 speed. This type of socket is called a ×8 (×1 mode) slot, meaning it physically accepts up to ×8 cards but only runs at ×1 speed. The advantage is that it can accommodate a larger range of PCIe cards without requiring motherboard hardware to support the full transfer rate. This keeps design and implementation costs down.