The common argument against a forth spec for hunters is that nobody has four dps specs. You could say the counter argument is that Blizzard never turned a ranged spec into a melee one before either. However it is unlikely that they would give the class four dps specs. The way to do four specs then is to make a healer or tank spec. There are also a large number of people who want the old SV back because they like the whole mobile dot feel of the spec. I also dont want to take away from those who want to play a melee dps spec either, but I think this could satisfy all these parameters. I am only going to go into general concepts as I have not played WoW since they turned SV into a melee spec, so I wont pretend I am up to date with current mechanics or abilities.
Marksmanship: Based around hard hitting but more stationary gameplay. Would have the abilities with cast times, like other spellcasters, but shots would hit very hard. For utility it would have the most abilities to keep an opponent snared to allow him to move away. Can have a pet but it will have a lower damage modifier than other dps specs.
Survival: Becomes a tank spec, which is hte best fit for the spec name. Pet goes from doing dps and providing raid utility to providing the SV with buffs. So if you were SV then different pets would do things like place a hot on you, buff your defense or hp, sacrifice itself to rez you,... Its taunt would also transfer the aggro to you. Abilities like Harpoon seem well suited for a tank so making the spec a tank spec seems a natural evelution.
Beastmaster: This is the trickiest to implement, as the spec would allow you to choose between playing melee or ranged, but your pet is still going to be a greater proportion of your dps than with other specs. Pets would act like they currently do but would have a higher base modifier for their dps. The spec would get most of its abilities through spells as you level up, but most would have lower dps by default. A bunch of SVs melee dps abilities would be transferred over to BM as Sv would be getting some new tanking abilities. Abilities that primarily affect your pet like Beastal Wrath would stay the same no matter how you chose to play.
Whether you play melee or ranged would depend on what talents you choose. So rather than talents that give you new abilities you would have talents that buff spells. So if you wanted to play melee then you would choose the talent that buffed melee abilities and if you wanted to play ranged then you would choose the one buffed ranged abilities. Blizzard could add some new abilities that are designed to interact with existing spells, so for example They could put in a shot that is buffed if fired after Cobra Shot, or an ability that is buffed if used after Mongoose Bite.
Woodsman: The new spec, which would act like the old SV but taken to its fullest extent. The spec will revolve around staying mobile keeping dots on the target. Pet would have a normal damage modifier and act the way pets do now. It would also have the most cc options with things like Freezing Trap. So most dot abilities like Black Arrow, Serpent Sting, and Barbed Shot would be part of its lineup. I would like to see Explosive Shot be a signature shot for the spec and have its mechanics interact with some of the other dots.
That is my idea for doing a four spec Hunter and giving it something other than another dps spec.