From UK NEO Report Chapter 3: Environmental Effects On Earth

"Tsunamis

Some two-thirds of the Earth’s surface is covered by the oceans so that the chances of an impact are greater there than on
land. As in the case of impact on land, a “crater” is produced in the water but such craters are unstable and rapidly refill. A
30 kilometre crater penetrates through all but the deepest ocean depths.These flows create a series of deep water waves, socalled tsunamis, which propagate outwards from the point of impact. Such waves are also set in motion by earthquakes and underwater land slips.They can travel far around the Earth with devastating effects. For example the 1960 earthquake in
Chile caused a tsunami in Japan, some 17,000 kilometres away, that killed at least 114 people. The maximum height above sea level in Hawaii for the same event was 15 metres, with 61 deaths.

Tsunamis travel as fast as aircraft, and their destructive effects can be enormous.The destruction is caused by the amplification in the height of the waves as the waves approach the shoreline. The inflow and outflow of the water mentioned above causes huge damage to property as well as risk to life.The evidence of the effects of large tsunamis, in terms of relocated rocks, is found widely, the most extreme example being in Hawaii where unconsolidated coral is found at 326 metres above sea level. Asteroid impacts are capable of producing tsunamis much larger than that associated with the 1960 earthquake, and may occur anywhere in the oceans.Tsunami from the Chicxulub impact deposited material widely and often far inland; recognition of such deposits in Haiti,Texas and Florida helped to confirm the nature and location of the event. The tsunami generated by the Eltanin impact about 2 million years ago is shown on the two maps on this page.

Objects that are small, or small fragments of larger objects that break up in the atmosphere, do not usually reach the Earth’s surface. But for objects in the range of between 200 to 1,000 metres, tsunamis may be the most devastating of all the consequences of an impact, because so much of the Earth’s population lives near coasts. Some studies have indicated that an impact anywhere in the Atlantic of an object 400 metres in diameter would seriously affect coasts on both sides of the ocean by tsunamis as much as 10 metres or more in height at the shore line."

So MN4 would have produced 10 meter waves on both sides of the Atlantic. How big were the ones from the 9.0 earthquake? Should have been bigger than the 15m event in Hawaii I think given the near record strength of the quake, but I know it depends on shifting geometry more than quake strength.

Regards,
Mark