Excellent sounding amp sim with circuit schematic tweaking and 3rd party vst/au loading, but lacks some of the features of previous major version. It feels amazing to play through but can require lots of fiddling and tweaking for optimal sound in a mix, however when tweaked just right, Revalver can also sound amazing in a mix. The free version has some modules available to try and extra amps and effects are purchasable individually and can even be demoed with saving disabled and occasional noise. It's definitely worth trying the free version since Revalver 4 isn't for everyone. Especially recommended for people that really like to tweak and fiddle.
Sound: Revalver 4 is based on SPICE-like circuit emulation and sounds quite different from a typical amp sim. It sounds more compressed and has smoother highs in a way that makes it sound less bright and spiky than a typical amp sim. This makes it feel quite realistic and organic but can also be perceived as muddying the sound somewhat. The natural compression does bring the guitar track forward in a mix and make it sound louder, but without tweaking, the slightly rolled off highs can make it sound duller than other amp sims (slight high shelf eq boost works really well). Either of these characteristics can be changed somewhat through tweaking, ranging from simple right click setting selection to opening the schematic and tweaking values, which can be a a steep learning curve but very rewarding.
Features: Amps, stomps, cabs, effects, kemper-like profiling amp (not free), circuit tweaking, vst/au loader, input can simulate other guitars. Although Revalver doesn't have as many models available as some other amp software, the ones it does have sound great, and there is a decent amount of variety and the circuit values can be tweaked. Also note that a few of the non Peavey amps have most of the circuit schematic hidden and uneditable, but fortunately the most useful Revalver output section parameters that are common to all the amps are still there. Most of the built-in speaker cabs have limited mic placement options available but there is an IR loader, and even better a module to load any 3rd party VST or au plugin. One cool feature is that the first RIR2 cabinet after the amp will interact with the amp's output section even while bypassed, so the effects of mixing and matching different amps and cabs on the amp's response can be used even with external IRs or speaker sims.
Usability: The interface is nice and compact and separated into tabs for stomps/pres, amps/cabs, and post effects, and each section can be bypassed as a whole. The preset management is a little clunky, but not unusable. There is something called gig mode meant for live use in which multiple rigs can be set up and switched between seamlessly via MIDI messages. Some improvements since version 3.5 but also some features removed.
Important changes from version 3.5:
Added amp/speaker impedance interaction with RIR2 Module, which also works while RIR2 is bypassed. (can be reduced or disabled in amp's tweak page near the output transformer, green square called "fSpeakerInteraction")
Way more in depth circuit tweaking (but less intuitive for beginners)
Amps can't process stereo, despite right click menu implying they can.
No longer has separate preamp and poweramp modules and no more single triode tube module.
input tone matching: can simulate other kinds of pickups.
Profiling amp (kind of like kemper)
TL/DR Sounds great, but not for everyone, try free version first. Feels great to play through, may need tweaking to work in a mix. Smaller selection of models but tons of tweakability, 3rd party VST au plugin loader.