Instead of I couldn't be any prouder of what I see use of what I am seeing so that it is clear you are using present tense.

They're all technically fine but potentially awkward. For example, in the first sentence, the person would probably just say "I'm so proud of you" unless it was something that had just happened and the person being spoken to was coming in from somewhere. Example: if a child were coming off of a stage or coming into the stands from a sporting event after having made the winning play the parent might say that. But that's about it. The second is just a past tense of the first but its awkward because the implication is the person speaking was proud of them in the past but they may not be now. An English speaker might say this in a lecturing tone i.e. "I was so proud of you but I'm not now." For the third, an English speaker would just say "I couldn't be prouder." The "of what I see" part would be understood. The fourth is fine. Hope this helps.