Both versions , Pianoteq 3 and 4, can be installed at the same time. You can also keep Pianoteq 1 and 2 if you like old stuff
Performance-wise, there should not be a major difference with pianoteq 3. On my own netbook with a 1.6GHz N280 Atom, Pianoteq 4 still runs as well as Pianoteq 3 with these settings: buffer size 192, internal sample rate 29400, polyphony 48 and multicore rendering turned on. I'm not saying that there won't be cracklings when you're banging the keyboard, and you have to make sure that no other application is running at the same time (firefox , anti-virus , etc..), but it runs decently well. And also make sure that the energy saving settings are set on 'Performance' mode, to keep the processor at full speed.
In fact the default preset uses less cpu because it uses only two mics, while the default preset of version 3 (K1 Solo Recording) uses 4 mics.
Regarding the reverb: the old reverb is still available, it is called "legacy" in the Ptq 4 reverb presets.