if you have 2 in your hand can you trow both. do you need two hands to use it do to having to pull the pin?
and do you take a -4 to the DC
Yes you could take a full attack action and throw a grenade with every attack.
There is no action to 'pull the pin.' Figure they are smart grenades in some function. If you have quick draw, the throw weapons don't even have to start in your hand.
Yes they take the any penalty to the attack roll to the save DC.
In WW2 some soldiers were rumoured to pull out the pins with their teeth before throwing. In the far future things would have moved on. Now, maybe you need a PIN number.
I've been told that that is a great way to end up with ruined teeth and an entirely unimpressed (and unexploded) grenade. Can't say I've checked though.