At two I would encourage you to continue to rear face, though I'd imagine a little Roundabout is outgrown already. They were tiny. There are still Roundabouts being made, just now they're the 55, not the little 40 you have (and the 50 was around relatively briefly).
I agree that for a six year old a properly used booster is better than an improperly used harness. Can your six year old buckle the Nautilus in next to the Roundabout? If so, that's a pretty wide backseat and just about anything would fit.
If not, I'd look at the Graco Turbobooster, Clek Oobr, Evenflo Big Kid, Britax Parkway SG. They're all pretty narrow. They would fit the four year old properly as well, though I'm a bit nervous about a four year old in a booster full time. If his harness is done properly, I'd probably leave him for a while in the Nautilus.
I'd also just teach your six year old to buckle up properly. At six Piper could do up her Nautilus harness on her own and do it correctly. If your husband isn't good about it, see if your six year old is. Show them crash test videos so they understand the importance. That goes for boosters too, if necessary. I'll occasionally show Piper crash test videos again just to stress that this IS life or death.
Wendy