I really don't want to pester, and I see it is canon, and I'm aware thats imaginary Warhammer 40k world… but 4 Brigades in a Regiment? Historically it was 2 Regiments to form a Brigade and after Korean War Brigade and Regiment became nearly the same organisational unit with different name. Roughly in contemporary armies it is Brigade = Regiment.
Unless you are talking about the British army, where a Regiment is (and always has been) an entirely administrative unit. Instead the British army is organised in Battalions. A Regiment will have a minimum of 1, but can have several (the current "Royal Scottish" or whatever it is called has 5, though will be downsized to 4 + 1 company). Regiments are not combat organisations, and instead Battalions (from pretty much any Regiment) will be organised into Brigades (usually 4 in the modern army).
40k uses these terms in its own way, but it shares the British army's position that a Regiment is not a fixed descriptor of the size of unit. It used to be that they were from 2-6000 men (The Storm Trooper Regiment was the largest with as many as 10,000! Gasp!), roughly Brigade sized, but the important thing was they were roughly the same in combat capability. Now.. well, they seem to range from Battalion strength to division strength (the 7th Tallarn Regiment being roughly of that size… but then Forge World got that one wrong, allowing one Regiment to have significant artillery, armour and infantry assets organic to itself).