DIGITAL REALTY TRUST, L.P., DIGITAL REALTY TRUST, INC., AS GUARANTOR, AND WELLS FARGO BANK, NATIONAL ASSOCIATION, AS TRUSTEE SUPPLEMENTAL INDENTURE NO. 4 DATED AS OF JUNE 14, 2019 TO INDENTURE DATED JUNE 23, 2015 3.600% NOTES DUE 2029