This was written by Robert Lamm, who is a keyboard player and singer for Chicago. It's about trying to write a song, with the title referring to the time of day: either 3:35 a.m. (25 to 4) or 3:34 a.m. (26 to 4). Lamm explained on The Chris Isaak Hour: "I was living with a bunch of hippies up above Sunset Strip. One of the advantages of this particular house was that it was in the Hollywood Hills and I could look out over the city late at night. I wanted to try to describe the process of writing the song that I was writing. So, 'waiting for the break of day, searching for something to say, flashing lights against the sky' - there was a neon sign across the city. That song came from the fact that it was 25 or 6 to 4 a.m. in the morning when I looked at my watch - I was looking for a line to finish the chorus.
Most songs that were written, especially in the early days, whenever I got them to the band and we started rehearsing them, that's when the songs took shape - once these guys got hold of them. There was definitely a lot of raw material, I thought it was a song when I wrote the words down, I wrote the changes down and I brought the charts to rehearsal, but it wasn't really a song until they all played it."
This quickly became a showcase song for Chicago's horn section, which featured on many of their hits from the '60s and '70s. Three of the founding members that have been with the band since its inception are trumpet player Lee Loughnane, sax player Walter Parazaider, and trombonist James Pankow
There are a lot of unsubstantiated rumors regarding the meaning behind this song's lyrics. A popular rumor is that "6 to 4" was a nickname for LSD, because if you dropped acid at 6 p.m., the effects of the drug would wear off by 4 a.m., 10 hours later.
This still gets a lot of play by college pep bands. The horns and tempo make it a great fit for sporting events.
Peter Cetera sang lead on this track. After he left the band in 1985, his replacement Jason Scheff took over vocal duties on this song.
The band was previously known as Chicago Transit Authority, which was the name of their first album. They shortened their name after the actual Chicago transit authority objected, and began releasing albums with their name followed by a roman numeral (Chicago II, Chicago III, Chicago IV, etc.). They did this throughout their career, even as they morphed from horn-driven rock to adult contemporary ballads ("Hard For Me To Say I'm Sorry," "Baby What A Big Surprise") in the '80s.
This is usually the last song Chicago plays at their concerts. In later years, Chicago would often tour with other high-profile acts that would join them on stage when they performed it. On their tours with Earth, Wind & Fire, this provided a great showcase for both band's horn sections, and was also a nice fit for Philip Bailey's vocals. Other acts that have joined them for this grand finale include The Doobie Brothers and REO Speedwagon.
Guitar World ranked this at #22 on their 2015 list of best wah solos of all time in praise of Terry Kath's use of a distorted, wah-driven guitar line during the second half of his guitar solo.