I have the Brenthaven Collins Sleeve Plus, which says designed for SP4. It fits the SP and type cover perfectly, and then has a small pocket with a sleeve for pen and another that holds the arc mouse. There is also room in there for the charging cable if you want to include it, but I usually go without it if I'm not going to be that long.
I've got a larger backpack that I use for longer trips, but I've not been on one yet. I do think it would get lost in there so I'm either going to try and find a smaller one; or just put the whole sleeve in my backpack.