- if i buy PS now can i use it on PC and PS4 with the same account?
Yes.
- would the save games be available in both platforms?
Yes... with an exception. You can't access any kind of DLC store or anything on the Windows app. That's fine, except when a game requires you to have DLC installed on both platforms to load a save. I tried to load up my Persona 5 save on PC when it came to Now the other day and it just won't load my save because it's 'missing required DLC'. It's because I downloaded some dumb free costume DLC like a year ago on my PS4. No way around it, I just can't play it on PC anymore. It won't let me load the save unless the DLC is installed which is impossible on PC. Yes, this is as stupid as it sounds.
- Can i use the xbone/360 controller on the PC version?
Yes. Unless a game requires the gyro or touchpad. But most touchpad actions are just pressing it (which becomes the 'select' button).
- In the PS4 can i choose between streaming vs download?
Yes. PS4 games (and the handful of PS2 games because technically they're PS4 remasters) can be downloaded or streamed on PS4. Stream only on PC.
- Do i need PS+ to play online even in the PC version?
You don't need PS+ to play PS Now games online. Even on PS4.