Exhibit 99.4

January 19, 2022

Dear [Valued Supplier / Vendor / Partner],

As you are a valued partner to Zogenix, I am writing to update you about some important news about Zogenix. Today we announced Zogenix has entered into an agreement to be acquired by UCB S.A., a global leader in developing and commercializing innovative therapies in neurology and immunology. The news release, which includes more information regarding the transaction, can be found at https://www.prnewswire.com/news-releases/ucb-to-acquire-zogenix-301463511.html.

UCB has a successful track record in epilepsy and is committed to addressing unmet needs of people living with specific or rare forms of epilepsy, and other rare neurological diseases. UCB is headquartered in Brussels, Belgium, employs approximately 8,000 people worldwide and its U.S. operations are based in Atlanta, Georgia.

We expect the transaction to close in the second quarter of 2022, subject to regulatory approvals and other customary closing conditions. Until that time, both Zogenix and UCB will remain separate companies and your contacts at Zogenix will remain the same. We will communicate important updates as information becomes available.

In the meantime, please do not hesitate to reach out to your Zogenix contact if you have any questions. As always, thank you for your continued support. We recognize our partnership has played an important role in our success, and we looked forward to continuing it.

Sincerely,

[NAME]

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Additional Information and Where to Find It

The tender offer described above has not yet commenced. This communication is not an offer to buy nor a solicitation of an offer to sell any securities of Zogenix, Inc. (the “Company”). The solicitation and the offer to buy shares of the Company’s common stock will only be made pursuant to a tender offer statement on Schedule TO, including an offer to purchase, a letter of transmittal and other related materials that UCB S.A. (the “Parent”) intends to cause Zinc Merger Sub, Inc. (“Merger Sub”) to file with the Securities and Exchange Commission (“SEC”). In addition, the Company will file with the SEC a Solicitation/Recommendation Statement on Schedule 14D-9 with respect to the tender offer. Once filed, investors will be able to obtain a free copy of these materials and other documents filed by Parent, Merger Sub and the Company with the SEC at the website maintained by the SEC at www.sec.gov. Investors may also obtain, at no charge, any such documents filed with or furnished to the SEC by the Company under the “Investors” section of the Company’s website at www.zogenix.com. INVESTORS AND SECURITY HOLDERS ARE ADVISED TO READ THESE DOCUMENTS WHEN THEY BECOME AVAILABLE, INCLUDING THE SOLICITATION/RECOMMENDATION STATEMENT OF THE COMPANY AND ANY AMENDMENTS THERETO, AS WELL AS ANY OTHER DOCUMENTS RELATING TO THE TENDER OFFER AND THE MERGER THAT ARE FILED WITH THE SEC, CAREFULLY AND IN THEIR ENTIRETY PRIOR TO MAKING ANY DECISIONS WITH RESPECT TO WHETHER TO TENDER THEIR SHARES INTO THE TENDER OFFER BECAUSE THEY CONTAIN IMPORTANT INFORMATION, INCLUDING THE TERMS AND CONDITIONS OF THE TENDER OFFER.

Forward-Looking Statements

The statements included above that are not a description of historical facts are forward-looking statements. Words or phrases such as “believe,” “may,” “could,” “will,” “estimate,” “continue,” “anticipate,” “intend,” “seek,” “plan,” “expect,” “should,” “would” or similar expressions are intended to identify forward-looking statements. These forward-looking statements include without limitation statements regarding the planned completion of the transactions contemplated by the Agreement and Plan of Merger dated as of January 18, 2022 by and among the Company, Merger Sub and Parent. Risks and uncertainties that could cause results to differ from expectations include: (i) uncertainties as to the timing of the Offer and the subsequent Merger; (ii) the risk that the Offer or the subsequent Merger may not be completed in a timely manner or at all; (iii) uncertainties as to the percentage of the Company’s stockholders tendering their shares in the Offer; (iv) the possibility that competing offers or acquisition proposals for the Company will be made; (v) the possibility that any or all of the various conditions to the consummation of the Offer or the subsequent Merger may not be satisfied or waived, including the failure to receive any required regulatory approvals from any applicable governmental entities (or any conditions, limitations or restrictions placed on such approvals); (vi) the occurrence of any event, change or other circumstance that could give rise to the termination of the Merger Agreement; (vii) risk that the milestone specified in the CVR Agreement is not achieved, (viii) the effect of the announcement or pendency of the transactions contemplated by the Merger Agreement on the Company’s ability to retain and hire key personnel, its ability to maintain relationships with its customers, suppliers and others with whom it does business, or its operating results and business generally; (ix) risks related to diverting management’s attention from the Company’s ongoing business operations; (x) the risk that stockholder litigation in connection with the transactions contemplated by the Merger Agreement may result in significant costs of defense, indemnification and liability; and (xi) risks and uncertainties pertaining to the Company’s business, including the risks and uncertainties detailed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 and its other filings with the SEC, as well as the tender offer materials to be filed by Merger Sub and Parent and the Solicitation/Recommendation Statement to be filed by the Company in connection with the tender offer.

You are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof. All forward-looking statements are qualified in their entirety by this cautionary statement and the Company undertakes no obligation to revise or update these statements to reflect events or circumstances after the date hereof, except as required by law.