In September 1939, the brigade consisted of three infantry battalions—the 22nd, 29th and 46th Battalions—and it was initially assigned to the 3rd Division. Following Japan's entry into the war in December 1941, however, the 4th Brigade was mobilised for war service and raised to a higher state of training at Bonegilla, Victoria. At this time the brigade was assigned a reserve response force role, and in March 1942, the brigade moved to Warwick, Queensland, and from there to Chermside to take up defensive positions around the north of Brisbane. Later in the year, the brigade was reorientated to defend the southern approaches to Brisbane. Later in the year, the brigade moved to Forest Glen, Queensland on the Sunshine Coast. In March 1943, the brigade was transferred to the 5th Division and it was deployed to New Guinea, being dispatched to Milne Bay to relieve the 7th Brigade. Later, the brigade took part in the Huon Peninsula campaign, and it was amongst the first of the Militia brigades to see action against the Japanese at Guiska. At that time, the brigade consisted of the 22nd, &lt;a href="29th/46th%20Battalion%20%28Australia%29"&gt;29th/46th&lt;/a&gt; and &lt;a href="37th/52nd%20Battalion%20%28Australia%29"&gt;37th/52nd Infantry Battalions&lt;/a&gt;. Throughout 1943 and 1944 the brigade was transferred between the 5th and 9th Division a number of times, taking part in the New Guinea campaign.