EXHIBIT NO. 99.(j) 4

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this Post-Effective Amendment to Registration Statement No. 2-50409 on Form N-1A of our report dated June 12, 2026, relating to the financial statements and financial highlights of MFS Corporate Bond Fund, MFS Limited Maturity Fund, MFS Municipal Limited Maturity Fund, and MFS Total Return Bond Fund, each a series of MFS Series Trust IX appearing in Form N-CSR of MFS Series Trust IX for the year ended April 30, 2026, and to the references to us under the headings “Financial Highlights” in the Prospectuses and “Independent Registered Public Accounting Firm(s) and Financial Statements” and Appendix M – "Recipients of Non-Public Portfolio Holdings on an Ongoing Basis" in the Statement of Additional Information, which are part of such Registration Statement.

/s/DELOITTE & TOUCHE LLP 

Deloitte & Touche LLP

Boston, Massachusetts

August 26, 2026