Correction: Sometime soon.
If I were inviting a woman out for a nice dinner, I would not use "grab." If I were to use that expression, I would use it more for eating at a fast food type place rather than a nice restaurant. If I did use it, I would use it more for a situation where perhaps we are already doing something together, sightseeing or whatever. "Are you hungry? Why don't we grab a bite to eat?" In other words, it's better used for spur-of-the-moment type situations where little or no notice has been given to the potential invitee.
Also, if I were inviting someone out, and I was really eager for it to happen, I would name a definite time. I would say something like: "Let's go out to eat. Could go this Friday night?" "Sometime soon" leaves it hanging.
Your first three answers are more appropriate in my opinion (keeping in mind what I said about "sometime") for asking someone to what you call "a nice dinner."