The nature of the 5'er you tried didn't suit you. Neck dive is neck dive and not a 5'er only, nor 5'er frequently thing. Any bass can have neck dive - and I agree that is not a good thing and I'd never buy a bass that didn't balance properly.
(Etc. about the neck dive comment I made...)
I see where you mistook what I was saying. It wasn't that 5 string basses neck dive and 4 string basses don't. It's that many of the basses I tried (4 and 5) were not well balanced, which disqualified them from my selection pool. To build on what we we're both saying, there were some 5 string basses that balanced
much better than some of the 4 string basses.
As for feel... That's a tough one. What you said about our not knowing much about how the instrument should fit,
technically, is definitely true. I suppose what I was saying was mostly about the instrument just being comfortable. It's hard to describe - much like it's hard to describe the differences in how different basses can sound. It's easier just to hear the difference rather than match the sound properties against a big list of technical specifics. For comfort, it's best just to pick the instrument up and see for yourself rather than measure, weigh, etc. Just pick it up and hold it.
I don't think that my comfort advice is bad at all. That carries over into many instruments. I've played and taught on drums for many years. The best way to select drumsticks is to try out a bunch of sizes. Go figure I use 7A sticks (which are pretty thin). I like a thinner bass neck, apparently. That's what works for me. Bad advice would be to ignore something that feels wrong simply because you're not supposed to know what it should feel like.
I definitely agree with your suggestion to take an experienced bassist along for the instrument selection process. I really don't have any bassist friends. The guy from which I had been borrowing a bass bought his instrument and never learned to play it. That's why he let me borrow it for so long! Fortunately, there are really helpful people that I've come to trust where I bought my bass. (I've dealt with them for pro audio, percussion, and synthesizer equipment purchases many times.) The salesperson I asked for was an experienced bassist. He spent a lot of time with me and answered tons of questions. Still, if I would have had a bassist friend available, I would have made them go with me.
Cheers!