1. yes, you can upgrade and you will have both v1 and v4 available on same computer (unless you uninstall v1)
2. if you're asking about 'will my songs open correctly in v4', there were some incompatible changes that you may face, so your songs might need some work to sound in v4 the same as in v1. and after you edit them with v4 you won't be able to open them with v1.
my advice: make a copy of each song before editing it in v4, this way you will have a copy that you can open in v1 until you're happy with them in v4.
