is your drive formatted as GPT ?
There's a bug in current version of the loader which can't load multiple partition if they are not identical. (or maybe if partition numbers on the disc are odd instead of even)
GPT FAT32 + NTFS = crash
GPT FAT32 + FAT32 = works fine
But you said it has 2 FAT32 ?
Maybe your drive has a hidden partition which makes odd partition count?
you can use MBR to fix the problem, or format both (or more) partition using the same format. or format a single 4TB FAT32 partition.
Just a thing I want to know :
Why do you want to use the "multiple partition" option ?
Do you know its purpose ? because I'm pretty sure almost nobody knows.
hint : NO, it's not used to enable support for more than one partition, or have Wii + gamecube on different partitions, that option is rarely needed unless you already filled 2TB of wii games on the first partition and you start adding Wii games to the second one, in that case it's normal it crashes if you set the first partition as "install partition" because it's full, you need to set the install partition to the second one (where you have free space).
the "multiple partition" is used for Wii games only, to list multiple /wbfs/ folder at the same time.