I wouldn't really recommend the explorer in general. The offset column reduces stability, which only gets worse the more extreme the camera positioning gets. You want to keep the camera load on top of the tripod as much as possible, not hanging out on the end of a (wobbly) boom. For some very specialized or hardcore macro users, it may be a good bet for convenience, but not for the general user and not as a general tripod. Personally, it also looks way too fiddly to me.
Gitzo 0-series is way, way too small/flimsy for your needs if you are thinking of putting a 400mm lens on it. I'd skip this one. Don't forget it's not just about load ratings, but really the key factor here is torsional rigidity. A small tripod can easily support a large weight, but cannot hold it steady in the torsional axis. That's where larger leg diameter really comes in to play. Gitzo only recommends 2-series tripods to be used with up to 200mm lenses, 300mm is doable but not optimal.
Have you thought about the new 2-series systematics from Gitzo? Those are likely to be "more stable" than the mountaineers. If I were buying a new Gitzo today I would most likely end up with a systematic and not a mountaineer or explorer or traveler.
Re: 3 vs 4 sections- AFAIK the consensus is that among modern, well-designed tripods, 4 sections is no less stable than 3. It's really more a matter of operating convenience. If you want a 4-section for compactness, don't worry about it being less stable.
PS Gitzo's numbering system is a little weird and at times inconsistent, but here are the basics. However it only describes the parameters of the tripod legs, not necessarily the model like mountaineer or systematic.
2-letter prefixes (usually) indicate the design. GT for mountaineers or travelers, GS for systematic. I don't know if it is still like this though.
First digit: "series"- basically indicating diameter of the leg tubes. Larger number is bigger diameter
Second digit: material. 3 for alloy, 5 for CF, 9(?) for basalt
third digit: number of segments per leg
fourth digit: model revision number- newer designs may have slight modifications but the fundamental leg geometry, etc. will be the same. One of the leg tubes on my 2531 has "2530" written on it because the tubes are the same.
2-Letter suffixes indicate special designs. Like OT for ocean traveler, EX for explorer, LS/XLS for tall/extra extra tall, etc.