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4v - allows optimized plug location which allows higher compression ratio,
allows greater valve area for better breathing / volumetric efficiency,
reduces valve train mass for each valve spring to control, which raises valve train rpm limit

25-40% more airflow.
Almost always cross flow.
Less "curtain" regarding valves
Automatically makes it hemispherical (or semi-hemi)
Valves are significantly lighter allowing for less valve spring tension which reduces frictional losses. Higher engine speed because of this.
Total cam drive load can drop more than 50%.
Less stress on camshaft drive equipment.

Forces center chamber plug location (single plug heads) Reduces "burn time" allowing less timing to reach MBT.

Normally smaller valve stem diameter and less side loading on valve. Reduces guide wear.

Only downside that I can see is added complexity. This can make shim style valve adjustments a PIA.
Many Toyota V8 have 32 valves to adjust!

Incoming charge increase
Easier exhaust scavenging

Turbos also force more air in
But more valves reduce NA restrictions for intake

More air in and more air out = more power

Check out the Ferrari 355 developed in the mid 90's. 5 valves/cylinder