I did the last half of my 400k with someone (V), first half alone.

I made a point of finding someone to ride with (3 people) between 11 pm and 4 am during my 600k. But I went into it completely alone. I might just as well have been completely alone for the whole thing, and in fact that stretch was really the only part where I had company. I just focused in on making friends during the ride. (Sounds a little like your 300k experience where you focused on finding someone to ride with.) That stretch I was very concerned about the sort of people I might encounter (drunk people, meth labs, etc.) and it was completely out of cell range.

But depending on the terrain, how comfortable I felt with the area, etc., I would consider doing a 400k alone. Devil Mtn Double (with 20,000 feet of climbing) took me exactly the same amount of time as my rolly terrain 400k (well, I finished both at right about 2 am), and I did DMD completely 100% alone (in 2007, as opposed to my DNF last Saturday). So I was ok with that.

There are plenty of reasons to put in the "to do" and "don't do" columns. Only til you start filling those columns out and really evaluating how you feel will you have your answer. How comfortable are you with the area? Is it in cell range? How confident are you in your equipment? Your mental health (only slightly jesting here)? Thing about those questions.