pitch shifting guitar to sound like a bass guitar in reaper?

Again, if we're talking about demos or scratch tracks you're better off sticking with pitch shifted guitar. If you want to use it in an official mix then a VSTi will be a much better choice. Personally I program each note by hand, so no piano knowledge involved. Chances are the playing won't sound as realistic on its own as a real bass (depending on the plugin and how good you're on programming), but the actual sound will be from a real bass so it will sit much better in a mix and give it the low end it needs.
 
Doing the things in piano roll format is very easy. Do you know the chords on the guitar? do you know more or less what notes are you playing?
Even if you dont know any of this, its still easy. Once you have the vsti loaded on your DAW, you just have play the chord or note you are doing and search for the same note in piano roll. Then you write 8 notes for compass. Its more than enough for a scratch track and for a metal song. It´s a bit harder only if you are writing music that have a prominent role in the bass, like funk or a jazz style or even a pop song with a good groove on the bass.
 
the ratio of opinionated elitism to actual helpful recommendations in this thread is staggering. if the OP asked which you prefer, i would understand, but nowhere in the title of the thread does it ask for your selfish opinion. so lets pump the breaks and check the ego just a TINY bit.....just a lil. tell the dude to stop playing music cause he wants to take some experimental and creative left turn? what the hell is wrong with you? what in your brain made you think that was the right thing to say in this instance?

now, to the OP. it looks like everything was covered at this point to push you in the right direction. pitch shift the track, like i said single coils sound better for faking a bass, and then just put it through amplitube. use the ampeg svt 4 pro, i prefer the 8x10 in the cabinet. tweak the settings. play around till it sounds like a bass. good luck, and good question. kudos for having the balls to ask.
 
I don't have Amplitube anymore, but I remember that there was a preset "guitar to bass" or "gtrbass", or something like that. Octave-pedal, bass amp, compressor and eq...

And before I had a real bass, I used guitar with Digitech RP255, there's one "gtrbass" preset too, and it sounds ok. At least a lot better than a robotic midi bass.
 
If you can programm drums than I don't see problem to write bass guitar notes for bass vst. Although I've used pitch shifted guitar I really prefer something like zombass over wanky pitch shifted guitar
 
I used to use a freebie bass vsti called EVM Bassline for a while back in the day, if you lopass it far enough you can't really pick out that its fake under heavy guitars. Imo that would sound more solid in the low end and require less fuckery to get it passable in a mix.