Nicholas. See please
https://www.volvoforums.org.uk/showthread.php?t=116489 This old but comprehensive thread re fuse 4 as well as fuse 6, fuel pump relay &c. &. will help you a lot. Bypassing fuses 4 and 6 with inline blade fuses is old hat and effective. Bypassing with MCB is just plugging MCB in to the inline blade fuse holders instead of plugging in blade fuses. Even I can manage that.
You and I discussed this when I installed two inline blade fuseholders and you deep cleaned the old style fuse board. Welcome to my preferred option now. Am I still, risk averse? 
.