I did a very similar setup with 4x AMD Instinct Mi50. For the air intake I used a single Noctua Industrial PPC 14cm fan, with a 3d printed duct. In the end the cards don't produce that much heat. A single 14cm fan at low speeds was enough to keep them cool at 62 degrees max. A large fan is also more quiet. Also, removing the graphene pad betwee the die and the heatsink, and replacing it with a good paste, helped bring down the temperature by few degrees.