The trick with MWF is that all four attacks are at your highest BAB, where as with TWF, only the first two are. That is actually quite a drastic increase in accuracy. A TWF may get more attacks in the end (up to 7 with all the feats and +16 BAB), but the iterative BAB means two of those attacks are at -5, two at -10, and one at -15 compared to highest BAB. That increase in the number of accurate attacks should be taken seriously, and is likely one of the reasons having multiple arms is on the expensive side.
However, the main hand, off hand penalties are the same as though you had the TWF feat, so you're still better off wielding light weapons in your three off hands to keep your accuracy up.
There is not Improved/Greatyer MWF. It seems Pathfinder purposefully did not bring that over from 3.5. That would make multiarmed characters extremely powerful. Can you imagine a monster with 6 arms, like the Marilith, getting itterative attacks with all it's off hands? Not a pretty sight for whoever's on the recieving end of that.
Having 3-4 arms also means you can, for example, wield a bow or crossbow in two hands, and a melee weapon in another, meaning you're always a threat no matter what range you're at and you always threate the spaces around you, without ever having to swap weapons.
Or, you can wield a reach weapon in two hands and a non-reach weapon in another, and a shield in the fourth. Now you threaten all the squares in a 10ft radius without the need for class features or feats with associated penalties and you have a shield to bump your AC to boot. And you never need to utilize MWF.
It also means that a wizard can carry a lot more and still be able to cast spells that require movement of the hands, and that might come in handy more often than you think.
So there are a number of options available to someone with multiple arms, both in combat and I'm sure out of combat. That increase in options in what you can do is why it costs the RP it does.
