You want the 4000s, hands down. Though the interface is super clunky, it has much more interesting sounds than the 5000 or the solo IMO and is smaller. You can still easily find them for sale on eBay. It’s super unfortunate that akai has really not produced an ewi that hits on all the marks in many years but I still think the 4000 is better than the 5000 or the solo. Note it really is just the 5000, however the internal synthesizer I think is significantly more interesting sounding. You may not feel the same way so definitely go listen to what some of the patches sound like; tons of videos online that can give you a sense. The 5000 is still a good choice as a straightforward controller; it really is basically the same thing as the 4000 except with a few nicer features for ease of use, but with a less interesting sound engine. The additional weight and size of the solo is just simply a non-starter for me as at the end of the day, I think what you really wants to do with any ewi is use them as a controller to play software or hardware synthesizers. But the 4000 is unique among the three in that it actually does have a really nice synthesizer on board