There are a myriad of capable lightweight heads and combos on the market right now. Finding the right one for you starts with narrowing the options.
What kind of music do you play?
How would you describe the tones you shoot for?
What's your normal EQ look like on the amp for your bass(es)?
How loud is/are your band(s)?
What size gigs do you play?
Will you have PA support?
(What do you like about the Mark IV/1820?)
First things to look at would be things like the [Invalid or Expired Link Removed] which can be found for better deals than the one I linked. Going to a music store and trying out as many amps as possible is always a good starting point to figuring out which ones are for you.
I moved in on a Genz Benz ShuttleMax 12 because it was one of the few heads to offer a strong EQ section, blendable channels with tube OD, more than enough power for any situation, and incredible size and weight savings all in one affordable amp to do it all for me. That said, I still adore my old '70 something Century Bass 200 head. It's probably 20 some pounds but sounds great. I'm still torn on what direction to go with lightweight cabs myself... fEarfuls are probably what I'll end up with later though.