I love my Boss BR 800 but I did not buy it to record guitar parts simultaneously. What it is good at is allowing you to overdub the various parts and layer your song instrument by instrument along with the vocals and create an optimally mixed and balanced end recording. You can use it to record a live performance and the built in stereo mics are excellent, but if that is all you wanted to do with it, you can find a much less expensive solution.
There is only one jack that is optimized for the input of an electric guitar or bass and that is on the front and labeled "Guitar-Bass (Hi-Z)". The 4 simultaneous input jacks accept either XLR Balanced input or 1/4" TRS or TS input either balanced or unbalanced and are suitable for microphones or keyboards. I have not tried plugging in a guitar to one of the 4 inputs on the back but the manual does not suggest that it should be done.
The effects cannot be controlled individually for each of the 4 inputs during monitoring of recordings but if you record 4 instruments simultaneously on 4 tracks, you can apply effects separately to each track during mixdown and mastering.
EDIT: I have never tried to record simultaneously from four inputs but apparently when you do, each input will be recorded to an individual separate track. When you select simultaneous recording "Input 1-4"from the Input Select menu, 4 tracks light up to indicate that the input from each input jack will be recorded to a separate track to allow different effects and EQ and volume settings to be applied during mixing.
You can record two inputs such as a mic and guitar to two separate channels by selecting stereo. Then you can adjust the pan for each input prior to recording if you want each of the stereo tracks to feature one of the two sources.
The BR-800 is a home studio multi track recording device that is optimized for creating recordings of songs or musical compositions. It's perfect for me to make demos of my original songs because I can record the rhythm guitar part first using the guitar input or the built in mics to record my acoustic guitar. Then I can overdub the vocals on a separate track. Then I can add lead guitar on a third track. Before laying down a rhythm guitar track - I can select a drum pattern to serve as a click track to keep the timing in sync with each overdub. Later I can customize the drum track with fills and intros and transitions and variations to the pattern at different parts of the song.
The only thing I ever do with simultaneous recording is vocals and guitar at the same time. After I have recorded all my tracks, I can apply effects to each track during the mixing and mastering process. I can also adjust the volume balance between each track and I can overdub over any mistakes. So it's really great for creating a good demo of a song because you can tweak each part until you get exactly what you want and easily convert the end result into an MP3 or WAV file.
But for simply recording jam sessions featuring multiple guitars, a digital mixer with a recording output might be a better solution. Of course you could input the output from such a mixer into the BR-800 if you wanted to - but I don't see any point in doing that because if you use a mixer with built in effects, you can do exactly what you described by assign different effects to each input channel.
But if you want to make demo recordings and compose songs or musical works layer by layer - the Boss BR 800 is a great solution.