The 4 had new buses only a few years ago. Not sure if that was just to temporarily fill in for the E500s which had did the 4 and 75 regularly or whether they wanted to upgrade it anyway. Then they all went to Express services and it has B9TLs and B7TLs mainly.
When the E500s arrived, they were due for the 62, Faifley to Baillieston. (Current 2).
Either when they arrived or just before, they realised that Parkhead depot didn’t have an Adblue tank (major planning error there) so they were put to Larkfield. They were put on the 75 however after a few months, they were getting majorly vandalised in Castlemilk, window damage etc and then a seat was set on fire which made them remove them from the 75 and put on the 44/A (current 4/A) as it went through areas of Glasgow where it was less likely to be vandalised.
After a few years they started to run again on the 75 as well and latterly the 5 too. The 44s/ 75 / 5 tended to then have E500s or B9TLs.
They were limited to the busy services they could put the E500s on as there were major works on the railway bridge at Cathedral St which had a weight limit meaning, as it was such a heavy bus, it couldn’t go on likes of the 38 or 57.
The 5 lost the New E400s after a while and went back to mostly 54plate B7TLs.
The 4s only got a few of the spare 65 E400MMCs on it, before they went to the express services, and also often saw some of the low height E400s (11/61 plate) too. (Which should be on the 6).