In 1994, happy hardcore basically consisted of sped-up breakbeats combined with piano riffs and pitched up vocals. By the end of that year, hardcore DJs such as Slipmatt were mixing techno tunes into their sets and some hardcore tracks such as those from the SMD stable started to appear with an enhanced 4/4 kick drum.
During 1995, UK producers jumped on this sound and combined the big kick drums and offbeat techno synth stabs with the breakbeats and pianos. An early example of this crossover sound includes Ramos & Supreme's "Life Force Generator", but it was "Toytown" by Hixxy & Sharkey which really brought the sound to the attention of ravers and clubbers alike, and dragged 4-beat into the public eye.
In 1996 and 1997, 4-beat was at its peak, with a revival in large UK rave parties and splitting the hardcore scene further from the Drum and Bass scene. Up until around this time, 4-beat and Drum and Bass DJs played at the same large parties in the UK.
By 1998, however, the music had become very commercial, abandoning it's original simplicity for over-the-top vocal cover versions of old pop records. Some of the records were well made, but although the music had not reached the same peak of the original breakbeat hardcore around 1992, people still decided to cash in and produce poor records. Sadly, many people left the scene at this time.
Some of the original 4-beat producers, such as DJ Brisk and Force & Styles have since produced popular UK hard house and trance tunes under different names. Recently 4-beat has made a significant UK comeback, with a less commercial, more trance influenced sound.
Somewhat ironically, some of the most popular original 4-beat tracks have been covered by pop-dance artists, including Bang's "Shooting Star" and Force & Styles' "Field of Dreams". The Bonkers series of compilations from React music, which began in 1996 are still being produced to this day, provide a good indication of how the music has changed over the years.
4-beat DJs and producers of note: Slipmatt, Seduction, Dougal, Vibes, Sy, Hixxy, Sharkey, Brisk, Force & Styles, DJ Unknown