A third shot showed large antibody increases. I haven't seen a fourth shot being suggested. Here it was being studied. Omicron specific vaccines are being worked on and expected to arrive in a few months.
Antibody levels are transient and not very important. What matters over the long term is cellular immunity. We're just chasing our tails by focusing on antibody levels.
https://youtu.be/GklHGYY8vN8