In physical terms of material and logistics,
- 1st gen used muskets and statinary lines and columns; both sides generally were evenly balanced and neither very mobile
- 2nd gen had barbed wire, machine guns, and some artillery; they moved around more
- 3rd gen introduced tanks and airplanes (I haven't seen air power mentioned in the official references yet; it must be there somewhere), and longer range artillery; tanks increased mobility of firepower; airplanes provided remote detachment by which small plane crews could wreak proportionately greater damage
The world's sole superpower is now shy to put troops on the ground. Bitter experiences in Viet Nam and Mogudishu have not been forgotten. The U.S. now prefers its superior air power, which is incredibly destructive and nearly untouchable by opposing forces.
Sadly, the destructive power is too often wasted on civilian populations while the small target individuals (that "dead-or-alive" guy from the Carlyle Group) are still unseen and too swift to be targeted.
Failure to truly commit to the mission, by putting trained and trustworthy troops on the ground, willing to sacrifice their lives for the cause, resulted in the failure in Afghanistan by which those who demonstrated the most intense conviction to prevail, have done so. Of course, such troops need a proper cause to which they can commit. Palestinians demonstrate high level of committment to cause.
- 4th gen seems to cycle back to the earliest days of combatancy; using very mobile small groups without vast dependence upon vulnerable systems which can be easily targeted; using whatever weaponry they can find or invent against whatever vulnerable targets are sufficiently insecure.
The literature on 4th generation warfare admits to examining only "the modern era" beginning with the muzzle loading rifle as 1st generation.
The guerilla tactics which we see today are probably not dissimilar to those used throughout Europe and Asia prior to the invention of the rifle.
The two big differences in the 21st centruy are that the combatants are not equivallently equipped and don't have hardly any understanding of each others motives or inspirations; and that the world's sole superpower displays a dazzling distribution of insecure targets, and after [intentionally] provoking the world's assemblies of retributionists, proceeds to chase birds around the field with sledgehammers.
When it comes to sneaking around quietly in the dark, and pulling out your tent spikes while you're sleeping, you can call it 'terrorism' or call it 'fourth generation warfare' or 'zero generation warfare' or 'cheating' or whatever; but you're the one stuck immobile and blind in your own high-tech tent, defeated.