Single- or Multifunction?
Of course, a single printer or scanner or fax machine or copier will normally use less energy than a multi function device with similar performance. So, if you are absolutely certain you will need only that one function in your home or office, don't read any further.
For all others, consider that a multi-function device (MFD) will use less than 50% of the energy of a separate printer + scanner + fax machine + copier. This general rule not only applies to the smaller 'all-in-one' machines for the home or small office, but also for volume production in larger offices where digital document centres can save 50% over the separate pieces of equipment.
Xerox claims that a workgroup of 100 people can save about $2,000 a year (assuming an electricity rate of $0.18 per kilowatt hour) in electricity costs by replacing eight workgroup printers, 24 personal printers and 12 fax machines with eight digital multifunction systems.
Also in the home, the standby-power which is the main issue with imaging equipment in the home of an MFD will be much lower than the sum of the standby-power for all 4 appliances. So, if you can live with the performance specs (speed, wake-up time, etc.) the MFD is probably your best choice. Check out the ENERGY STAR database and find out how you can both save the environment and costs.