I started with B3, then MS-60B and now the Zoom B1-Four as of a couple of days ago. I would describe the order as Liked, Loved, & Floored in that order. Zoom has seriously improved the sound quality and seems to have listened to feedback and addressed the weakest aspect where people thought it lacked (in the OD/Dist area & the Tuner operation). Granted, I would have rather seen them stay with a metal case like the MS-60B, but inside there is a whole lotta new & a whole lot to like.
The B1-Four has KILLER distortions as compared to previous versions - and not just because they've modeled some newer pedals, they've even made some the older, familiar ones x1000 times more useful by adding new parameters to tweak (like a Filter characteristic to control the bite of the tone, and a Dry/Mix control on the ProCo Rat) among others.
The Cab sims have a lot more tweakable parameters like two types of mic placements (for cabs with tweeters) and Dry Mix, etc., in order to get your tone just right. In fact, almost all of the effects I'd mentally written off as "not so good" on the older units are greatly improved and highly surprising me and this applies to the Filter effects as well. So it's been awesome re-discovering the improved effects.
The Tuner is hugely improved upon - there is no "hold time" before it kicks in anymore - thank goodness. It's an instantaneous on, and it uses much easier to see Red/Green LED lights in addition to the note showing in the LCD panel. It also it tracks blazingly fast, even when you're moving & playing fretted notes up and down the neck - like on par with my Turbo Tuner ST-200. With the latest processor sounding this good and the addressed OD's & Tuner, this latest Zoom is a solid piece of gear worth looking at. Kudos to Zoom for listening to and addressing user feedback.
I can't think of a multi better suited for me to just throw it in my bass case and have the confidence that it's gonna sound excellent for the gig than the B1-Four. So for me and my needs, the B1-Four is the next step in life after the B3. I don't see myself needing to go down the route of +$500 processors anytime soon, but I know they're out there if I ever want to take that path.