Why Tiger could remain useful beside the G3s

Would Tiger still have a front-line role after four G3 battlecruisers entered service?

Yes; the G3s could face Japan while Tiger and the other existing battlecruisers protected Atlantic trade routes.

The arrival of the G3s would not make every older battlecruiser redundant. The newer ships could be concentrated in the Indian Ocean against the most demanding Japanese threat. Tiger, Hood, Renown, and Repulse would remain fast enough to protect Atlantic trade routes and deter the raiders likely to appear there. A global navy can keep an older ship useful by matching it to a less demanding theatre.