Why use four-inch guns instead of developing the mounting?

If increasing the gun's elevation was straightforward, why was it not done in time?

Working through the modification and testing it was expected to take about twelve months. Existing four-inch guns could start being fitted within days or weeks.

A workable geometric idea still requires development and testing before the fleet can rely on it. The estimate allows about twelve months to complete that work properly. Four-inch guns already in storage offered a known solution that could begin reaching ships within days or weeks. A proven 4.7-inch modification would reverse that calculation, because changing the existing mounting could then be quicker than supplying replacement guns. Rapid wartime adoption often depends on development completed before the immediate need arises.