How the Royal Navy became locked into the Tiger class

Was the Tiger class deliberately built as a large, upgradeable hull despite carrying only four six-inch guns?

No; the Royal Navy inherited and completed a programme it no longer strongly wanted, then adapted the ships later.

The question points to the apparent mismatch between displacement and the small main battery. The Royal Navy was not especially enthusiastic about the class. Existing hulls, work already committed, and the momentum of a light-cruiser programme narrowed the service's choices after requirements had moved on. Later conversion potential gave the ships another use, but it was not the original explanation for the inefficient layout. Sunk costs and inherited construction can shape a fleet more strongly than current design preference.