I’d say if you need a lot of large size renders in your work, the new noise reduction in V4 can be worth it (but there is a bug for now which makes it lose a bit too much detail than necessary), otherwise you can still use V3 for the time being. The noise reduction lets you run renders until SL 13 or so, instead of SL 19 for an interior for example, and save you lots of render time. You can have both V3 and V4 installed but you can run into issues with licensing so I wouldn’t recommend it.
Hopefully there is an update to V4 soon, but on the Maxwell forums one developer said this update would not yet have additive materials working on GPU so that would mean it’s still not usable for most production renders. Maybe it will fix the denoiser bug though. For now there aren’t any new major features in V4 except the denoiser. There is the new standalone MultiLight tool that comes with V4 and that might be useful to you for showing your designs to people.