Far from me to buck a trend, I concur with the consensus recommending the #4. It's the one I most often reach for in just about any circumstances.
Using a plane isn't rocket science and really doesn't have a huge learning curve. However, setting it up is a somewhat different story. Still not rocket science but make you adjustments and test them on scrap before you take on your panel.
For what you are talking about you will probably want the throat closed to it's maximum and the blade set for taking the very finest of shavings off. Don't get greedy and try to do it in one pass. Attack the surface with the grain but with the plane at a slight angle to the direction of travel. This will give you a cleaner slicing cut and help avoid chatter. Don't bear down on the plane, let it do it's thing mostly by weight. You're not trying to remove a lot of material here, just some raised lines.
When doing a panel you will probably find the hardest thing is getting the blade parallel to the sole of the plane so don't test you adjustments on the edge of a piece of stock do it on the face so you are taking full width cuts.
Good luck