Hi everybody! Thanks for the spirited discussion, it’s been really great and useful for me to read through it all. And thanks Horse for basically being a stand in for me during the discussion to get a clearer idea of what the issues are, since at first, I truly didn’t get it, but with the back and forth you guys have done, it’s finally sunk into my thick skull what the problem is 
(And now we interrupt this post in progress for an obligatory apology to the community)
Here’s the deal: I screwed up, and I’m sorry for it. Yes, I can totally see now the keyboard issue of limited thumb and pinky love, since not everyone has a mouse with a button 4. If I could go back in time, I would inform myself of the PC problem I’m creating with the all in one grenade button and steer the whole thing a different way.
The problem is now that the code changes that had to be done to our inventory management system to make the one-button grenade approach work means it’s unfortunately not an easy-peasy task to put in a traditional PC version in addition to the new system. Time is running short, and our gameplay programmers are stretched really tight, so I’m afraid this can’t go on their list of things to do to support both the new and the old system. Hence my sincere and profound apology. I was dumb, I missed the trick, and all I can do is say I’m sorry, lesson learned. 
HOWEVER, good news! I’ll double check tomorrow, but I believe Darkangel’s script solution from a couple of pages back is a perfectly viable way out of this sticky wicket, in that one could map a button to toggle the bound functionality of mouse 1. So you hit the button and bang, M1 is now grenade-ish. If you hit the button again, M1 goes back to rootin-tootin-shootin. Absolutely brilliant solution, and that should work fine.
So there’s our way to have our cake and eat it too. Phew, bullet dodged. I’ll also try to go one step further and see if, while we’re working on our control remapping option screen, there can be a “grenade button shift/toggle” choice that would basically activate that script, so PC players don’t have to bother with that. But it might be a case of players having to do the script themselves. Fingers crossed on that.
So, this whole episode has opened my eyes to the fact that, in spite of my best efforts, maybe I am still a bit too console-headed (tough professional habit to break, you must understand). That’s why I’m going to open the floor to discussion about it further. I’ll have to double check before I start a new thread on this topic, because to do the discussion right, I’ll need to get clearance to talk a bit about a game feature that hasn’t been revealed yet (it’s a little thing, and I’m sure it’ll be fine, but I still have to get clearance from the PR gods). Look for said thread in the next few days, were I basically lay out what our current thinking is for all the default PC keyboard/mouse control layouts, and get your guy’s feedback on it.
Thanks for all your help guys, and also for keeping this thread fairly level headed, even when it’s obvious tempers were close to starting to flare up. Good work all, we’ll get this game done yet! 