Letters of recommendation -- from postdoc or professor? Both?
Realistically, your acceptance into a good doctoral program will depend on an assessment of your probability of success in research. Funding might depend a bit on other things as well, but not the acceptance itself. Normally that would suggest professors are to be strongly preferred.
In your case, however, the post doc has some knowledge of your research potential and that is very important to get on the record.
If you can send four letters, then send them all. If you can only send three then you need to balance what people are going to write so that you can get a complete and positive picture of your research potential. Whether &quot;diversity&quot; helps or not comes back to what they will say about your likely success with research in your own field. But there is little downside when people praise you and your work.
The fact that you have some experience as a TA will only give them some confidence that you will work out as a TA provided you are accepted. It will have little effect, or none, on the acceptance potential.