The lowest 4 strings on the guitar are the same tuning as a bass, only a different octave. I've played the bass line for a number of different projects and bands on my electric guitar with the bass end turned way up on the amp.
However, the main difference is not in the notes... it's in technique and style, in playing thick strings with wide fret spacing, and learning to move between notes with a minimum of string noise. It's hard to mimic a true bass technique on a guitar. About the best you can learn is notes and progressions, which for some applications and starting out, is probably just fine. I'd leave all 6 strings on, though, to keep the tension on the bridge even. You could even tape down the two high strings with that blue painters masking tape (it doesn't leave a residue like regular masking tape.