Company: SION
Filing Date: 2025-02-07
Form Type: 424B4
Source: 0001193125-25-022709
Chunk: 121

Company: Sionna Therapeutics, Inc.
Filing Date: 2025-02-07
Form: 424B4
Chunk 121
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 cash needs. Therefore, our actual expenditures may differ materially from the estimates described above. We may find it necessary or advisable to use the net proceeds for other purposes. 86

We will have broad discretion over how to use the net proceeds to us from this offering and investors
will be relying on the judgment of our management regarding the application of the net proceeds. Pending our use of the net proceeds from this offering, we intend to invest the net proceeds in a variety of capital preservation instruments, including
short-term and long-term interest-bearing instruments, investment-grade securities, and direct or guaranteed obligations of the U.S. government. We cannot predict whether the proceeds invested will yield a favorable return.

87

DIVIDEND POLICY

We have never declared or paid cash dividends on our capital stock. We do not anticipate declaring or paying, in the foreseeable future, any cash
dividends on our capital stock. We intend to retain all available funds and future earnings, if any, to fund the development and expansion of our business, and we do not anticipate paying any cash dividends in the foreseeable future. Any future
determination regarding the declaration and payment of dividends, if any, will be at the discretion of our board of directors, subject to applicable laws, and will depend on then-existing conditions, including our financial condition, operating
results, contractual restrictions, capital requirements, business prospects and other factors our board of directors may deem relevant.

In
addition, our ability to pay cash dividends on our capital stock in the future may be limited by the terms of any future debt or preferred securities we issue or any credit facilities we enter into.

88

CAPITALIZATION The following table sets forth our cash, cash equivalents and investments in marketable securities and total capitalization as of September 30, 2024:

| • |     | on an actual basis; |

| • |     | on a pro forma basis, giving effect to (i) the automatic conversion of all outstanding shares of preferred stock into                                                                                                                              
 an aggregate of 27,149,206 shares of common stock immediately prior to the completion of this offering and (ii) the filing and effectiveness of our fifth amended and restated certificate of incorporation, which will occur immediately prior to 
 the completion of this offering; and                                                                                                                                                                                                               |

| • |     | on a pro forma as adjusted basis, giving effect to (i) the pro forma adjustments set forth above and (ii) the                                                                                                                           
 issuance and sale of 10,588,233 shares of common