Recently a friend has insisted that his warg riders throw their spears at S4, referencing the rule which allows them to substitute the warg's strength for the orc riding it. I had thought this meant when striking in combat only however, as the model throwing the spear clearly does not have S4, but when I checked the rules I didn't see anything saying he couldn't. Is my friend correct here?

No, all throwing weapon have S3 as stated on the missile chart.
You are correct, using the strength of the warg only applies to the combat strikes.

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Operation future in full swing.

As Bartelomeus said...

Besides, if your friend was right, then every hero with a throwing weapon would get an S4 (or higher) shot. Give Eomer some throwing spears and see how he likes it