ITUNES

I have a question hoepfully someone can help. Currently my wife and I have 3 apple devices all "sharing" her original i tunes account. Eventually we would like to add a 4th for me. My question is this on my current apple device (Iphone 4s) I have all of our music, is it possible to have my wn separate account for itunes yet not lose any of the music I have on the device? Also, if and when we add a 4th device am I able to downlaod all her/our music and then have my own itunes account afterwards? Thank you