My transmission vibrates for an instant when shifting into 4th gear. It feels like running on grooved pavement for an instant. It doesn’t happen every time and when it does, it varies in intensity from very slight to quite noticeable.
One possibility is a torque converter lockup shudder - under certain conditions your engine & transmission get “locked” together to spin at the same speed. Often an early first sign of a problem with this is that it rapidly locks/unlocks creating a shudder.
You should check your transmission fluid immediately. How many miles are on this vehicle and has the transmission been serviced regularly?
I am just guessing, so I wouldn’t go on my word a lone, but a visit to a good, local (NOT a corporate chain) transmission shop may be in order. Start by just having them scan it for trouble codes.
The Acura MDX has a somewhat common problem with the torque converter going out; drivers often report the symptom as feeling like driving over rumble strips. It sounds like you’re feeling the same thing, so unfortunately maybe it’s the same problem for your car.