The 4.5inch family of weapons began pre WW1 when a 4.5inch howitzer was used in artillery regiments to support the 18 pounder. Later, the US army undertook development of a 4.5inch weapon that would share design features and ammunition with a British weapon based on converted 60 pounder medium guns.
By the beginning of WW2, the 4.5inch Field howitzer still soldiered on supporting the 18 pounders, and were taken to France in 1939 with the BEF along with a few of the 60pounder conversions. All of these were left in France when we evacuated in May 1940. There were still numerous examples left over and pressed into service to defend the UK, but also in many parts of the world it still served.
In mid 1941 a new weapon had been developed and introduced which was the 4.5inch medium gun or 'Sniper' due to its good range. It first saw action during Operation Crusader in November 1941, and from then on it and it's sister, the 5.5inch medium gun saw service everywhere the British and Imperial army fought.
Thereafter the 4.5 inch weapon type became the preserve of the Royal navy until today.