I dont own Normandy yet, but I have read the rules and I think that open-top ability is a little bit unclear. I like the ability and it can be great fun. I guess the main effect of a open top ability is that the vehicle is a lot more wounderable to artellery and mortar fire. I geuss also that was the entire point of this ability, but it raises a few questions:

1: the vehicle still rolls 4 armor dices pluss any cover bonuses for defence right, even if firing with suppresive fire, right?

2: Can you use your tiger to fire a suppresive fire attack against that vehicle? That sound quit odd and realy strange.

3: Can infanteri fire suppresive against these vehicles? This does not sound that strange but...

4: I guess that all units still use their anti-vehicle fire stats and not their anti infanteri stats, even when fireing suppressive?

In reverse order:

Yes, all attacks made against vehicles with the open-top trait roll attacks as they would any vehicle.

All units can declare either a "Normal" or "Suppressive" attack.

Vehicles with the open-top trait still get to roll for armor and any additional cover (wooded, rough, bridges)