Categorization of fighter jets in generations is a classification system used around the world. The basic classification into five generations is widely accepted and recognized. Some accounts have subdivided the 4th generation into 4 and 4.5, or 4+ and 4++.

Four and half generation jet fighters (late 1980s and into the 90s)

The concept of having a half generation increment stemmed from a forced reduction in military spending at the end of the Cold War.

It became more cost-effective to add new improved features to existing platforms. Later variants of 4^th gen aircraft progressively enhanced their characteristic technologies and incorporated some of the emerging fifth generation technologies (such as high-capacity digital communications), leading them to be classified as an intermediate generation (4.5 or 4+ or 4++).

So 4.5 generation aircraft are basically fourth generation fighters with basic characteristics of fourth generation planes but with enhanced capabilities provided by more advanced technologies that might be seen in fifth generation fighters. Many of these types are in frontline service in 2021.

These aircraft have advanced digital avionics based on microchip technology and highly integrated systems. They are adapted to operate in high-tech warfare where not only avionic but also super manoeuvrability are the key to success.

Their features include stealth, radar absorbent materials, thrust vector controlled engines, greater weapons carriage capacity and extended range and endurance.

The addition of an Active Electronically Scanned Array (AESA) radar is a significant enough game-changing feature. The AESA radar allows fighter aircraft to perform a limited Airborne Early Warning and Control function.

Advances in computer technology and data links also allow these  fighter aircraft to be integrated into a network centric battle space where fighter aircraft have much greater scope to conduct multi-role missions.

According to available sources, fighters in this generation include the following:-

Boeing F-18E/F Super Hornet
Sukhoi Su-30
Sukhoi Su-33
Sukhoi Su-35
Eurofighter Typhoon
Saab Gripen
Dassault Rafale

Coming Up: Generation wise description (5th and 6th generation)

References:

https://www.airforce-technology.com/features/top-sixth-generation-fighter-jets/

https://www.sldinfo.com/wp-content/uploads/2014/05/Air-Combat-Seminar-summary-AndrewMcL.pdf

https://www.fighterworld.com.au/az-of-fighter-aircraft/five-generations-of-jets

http://www.codeonemagazine.com/archives/2005/articles/oct_05/

https://en.wikipedia.org/wiki/Fifth_generation_fighter

https://www.airforce-technology.com/features/top-sixth-generation-fighter-jets/?utm_source=Army%20Technology&utm_medium=website&utm_campaign=Must%20Read&utm_content=Image

https://migflug.com/jetflights/fighter-jet-generations/

http://www.aerospaceweb.org/question/history/q0182.shtml

Comments and value additions are most welcome