If none of the GPU overheats (75 degrees seems perfectly fine, even 80 would be OK), the only other reason I can think of that would cause them to down-clock is if they exceed the power limit, or if the power supply does not deliver enough power. nvidia-smi can show the max. power rating for the GPU as well as the current power draw.
What does not make any sense to me is the statement that the GPUs do not overheat at stock fan speeds, but nonetheless down-clock unless cooled more aggressively. Something is very wrong in that scenario, but I cannot remotely diagnose what it is. You also stated earlier that you saw GPUs go up to 85 degrees, how does that jibe with your information that “after hours” of operation they only reach 75 degrees? Power consumption will differ widely based on workload, are you quoting temperatures from two difference workloads by any chance?
As a sanity check, I would make sure that all the power connectors are plugged in, and make sure the power supply has sufficient output to drive four GPUs. With some power supplies you may need to take care how GPUs are matched to “rails”. My recommendation would be to use a power supply that is rated at 1.5x the combined peak power consumption of the GPUs plus the CPU. For example, if each of the four GPUs is specified for 235W max power, use a 1500W PSU.
How are Tesla cards cooled? It depends on whether you have an actively or passively cooled model. The actively cooled ones come with a fan, basically the same way as a consumer GPU. The passively cooled models have a heat fin assembly, and require that the fans in the server enclosure blow just the right amount of air over these fins. This usually means you need to buy such a system from an integrator that partners with NVIDIA so you can be sure the cooling is set up correctly.
Some adventurous souls have tried integrating passively cooled Teslas into their own systems, and often it doesn’t work right due to insufficient cooling. That does not mean it can’t be done, one just has to have the knowledge and experience to set this up correctly, and few people have that.