I think it depends on what type of saws and/or tools you have available to you. I would prefer to cut them to dimensional size, sticker 'em good so they don't bow/twist/warp, and leave them for a year.
If your using a chainsaw mill, I would agree, as it is a lot of work to cut the slabs into usable lumber. However, the slabs will be big and heavy, so if your thinking of ripping them on a table saw, for instance, you'll need to have the means to manipulate the slabs while you do that.
On a band saw mill, it's not as hard, stack the boards on end and cut 'em down. You might be able to make a jig to hold the boards so you can cut the width. IOW, slab multiple 2" slabs, then stand those on end and clamp them together, then setup the chainsaw mill for a 4" cut and cut about 6 at a time. This would be a fair amount of work with a chainsaw mill.
Your other option is as you mention, slabbing it up, and stickering it good, then rip it into 4" boards next years. I'm not sure if you have ever tried to rip a 8' slab that is 2" thick on a table saw, but that can be a job in itself. You might be able to do it with a skill saw and a long straight edge, but could only cut one at a time.
Food for thought.