Can’t remember the year but not so many years ago on a Sunday passing Lille on the way home from Paris to G(h)ent in Belgium there was a huge traffic jam in opposite direction. It was around 7 pm and think many were returning home from a weekend trip out of town. No idea if this is always the case on Sunday early evenings, especially more earlier around 4 pm. If balso says that 4 pm should be a quity time I believe him.
Like Nigel I have passing Lille many times, but this was an exceptional traffic jam, usual there is always traffic and sometimes slow moving. But think in general not a reason for serious concern. The train stations are close to the freeway (N356) I follow and the exits are busy, but never caused in all those years traffic jams or serious delays for me.
But to be sure to my opinion arriving a bit earlier can avoid unnecessary stress. An accident at an unlucky place and you are in trouble. Btw Lille is a nice place to visit so you can do some sightseeing after dropping the car and before catching your train. It’s historic center is an easy walk from both railway stations.