Adding a 4-way switch to a Tele is one of my absolute favorite mods. Without any visual differences to the instrument you get a totally different and really really useful 4th sound, namely bridge and neck pickups in series. Normally, when 2 pickup are combined in position 2 of a Telecaster, they are wired in parallel. This is a fine and very classic Fender sound. The nice thing about the 4-way mod is that you don’t lose this sound, as the classic parallel sound still remains in position 2. However, you gain this really beefy tone with the added series connection in position 4. It provides a noticeably thicker and louder sound that is a bit darker as well. In essence, you’ve taken the 2 single coil pickups and combined them to become a humbucker in this position. I like to kick this position in for solos.
The only drawback of this mod (which in my opinion is pretty minor) is the potential for a bit more added noise, but only when in position 1 (note: I refer to the “bridge pickup only” position as position 1). I will explain the reason for this noise. as well as a solution.