In the USA, 3 Phase 4 wire = the 4th wire is earth ground.
Not always, because MANY manufacuters define a "4-wire" system to be one with a neutral bus. Where there is a chance of misunderstanding, the power system configuration must be clearly defined.
Typically, there is no 3 phase neutral as there is no set standard what the voltage to it would be.
NO! This is simply incorrect. There are many power systems that do use a 3-phase neutral, and have a dedicated neutral bus. There are plenty of standards and codes that define what a neutral is.
There isn't a neutral in a three phase system, altough one leg (delta) or all legs (wye) are usually tied to ground for safety.
Careful...I estimated a system yesterday, 480 volts 3-phase, but with a 277 volt phase-to-neutral voltage, so that 1000 watt 277 volt metal halide lighting fixtures could be used. Sometimes a plant has a 3-phase system with a neutral bus but does not use it. Other times, the 3-phase panels have neutral buses that are used, but the electricans are unaware of it. If a 3-phase panel has a used 1-pole breaker, you can be confident that you do indeed have a 3-phase neutral.
Does 4-wire mean 3 phase plus neutral or does it mean 3 phase plus ground and no neutral?
Without more information, I would say "4-wire" as it is most currently used, means "3 phase conductors and a neutral conductor". Due to NEC grounding requirements, normally the neutral bus for a Wye secondary system would also be grounded (connected to ground).
EDIT: I just pulled out a Square D catalog and looked at Section 6, Panelboards. Square D defines a 3-phase 4 wire panel to be one with a neutral bus.