No, the deposition transcript isn't the only way to effectively cross-examine and discredit a witness' testimony.
A party has the right not to take any depositions, or to cancel depositions that have been noticed. This is not unethical.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.
If the deponent is before the court, deposition testimony can be used for any purpose. If the witness has not been called, the deposition testimony should only be admissible on a showing that the witness is "unavailable." Whether the witness is truly unavailable or just unwilling to come to court is key to whether the deposition testimony comes in. Don't see any ethical issues re: the four deposition notices.
I agree with my colleagues that there is nothing exceptional in the facts you describe. I agree that depo transcripts are a good source of impeachment information and can be used to refute a witnesses' trial testimony; "were you lying then or are you lying now?"
I have been licensed to practice in the State of Oregon since 1990. I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.