This thing is an absolute monster. It was purchased recently to replace our semi-demised ASR-10 (now doing duties as a keyboard controller only). We are still trying to get our heads around it at the moment owing partially to its relative complexity and partially due to the fact that the manual is extremely perfunctory. Lots of detail about what is there but very little about how to actually use it. Fortunately, we're slowly but surely getting our heads around it and it's very, very impressive.
64 oscillators going into multi-mode, resonant filters and to the amplifier section, which can be routed through any or all of 3 effects processors and four outputs. There is a host of modulation sources, including filter, pitch and amplitude envelopes, one LFO per (stereo or mono) voice and the fearsome Program LFO (YamahaSpeak for MIDI syncable LFO with extra features) which can act as a modulation sequencer with up to 16 steps.
The filters are very, very analogue sounding and the 4-pole filter can be pushed to self-oscillate. This all means that the A4000 can be used as a capable (but basic in terms of architecture) analog-style synthesiser. We're currently in the process of moving the old Ensoniq sample sets onto it as well as building up an all-singing, all-dancing sound library for it.
A fantastic piece of kit.