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Fourth generation aircrafts are the most popular aircrafts of all the times. The 4th generation got very popular because the designs are heavily influenced by lessons learned from the previous generation of combat aircraft. Long-rangeair-to-air missiles, originally thought to make dogfighting obsolete, proved less influential than expected precipitating a renewed emphasis on maneuverability. Meanwhile, the growing costs of military aircraft in general and the demonstrated success of aircraft such as the F-4 Phantom II gave rise to the popularity of multirole fighters in parallel with the advances marking the so-called fourth generation.
Aircraft classified by the United States government as fourth-generation jet fighters are those in service approximately from 1980 to 2010, representing the design concepts of the 1970s.
During the period in question, maneuverability was enhanced by relaxed static stability, made possible by introduction of the fly-by-wire (FBW) flight control system (FLCS), which in turn was possible due to advances in digital computers and system integration techniques. Analog avionics, required to enable FBW operations, became a fundamental requirement and began to be replaced by digital flight control systems in the latter half of the 1980s.
The further advance of microcomputers in the 1980s and 1990s permitted rapid upgrades to the avionics over the lifetimes of these fighters, incorporating system upgrades such as AESA, digital avionics buses and IRST. Due to the dramatic enhancement of capabilities in these upgraded fighters and in new designs of the 1990s that reflected these new capabilities, the US government has taken to using the designation 4.5th generation to refer to these later designs.
This is intended to reflect a class of fighters that are evolutionary upgrades of the 4th generation incorporating integrated avionics suites, advanced weapons efforts to make the (mostly) conventionally designed aircraft nonetheless less easily detectable, and trackable as a response to advancing missile and RADAR technology, see stealth technology. Inherent airframe design features exist, and include masking of turbine-blades and application of advanced sometimes radar-absorbent materials, but not the distinctive low-observable configurations of the latest aircraft, dubbed fifth-generation fighters or craft such as the F-117 and B-2.
The United States Government defines 4.5 generation fighter aircraft as fourth generation jet fighters that have been upgraded with AESA radar, high capacity data-link, enhanced avionics, and “the ability to deploy current and reasonably foreseeable advanced armaments
For more details check my previous post https://hassaanrabbani.wordpress.com/2011/08/05/the-legendary-f-16-fighting-falcon/ you will get to know about “relaxed static stability” (RSS) and many technicalities of 4th generation fighters
for some advanced technologies of 4th gen combat airplanes.