17-4PH

ASM HANDBOOK Volume 6 Welding, Brazing, and Soldering has some information on this subject ( For thicknesses in the range of 23-60 mm and a preweld condition of H1150 (very similar to the H1100 you mentioned), mechanical properties are listed for two conditions: one with no PWHT and the other with a typical ST (solution treated) + aged at 595 C for 4 hours.

PWHT YIELD TENSILE ELONGATION
None 705 MPa (102 ksi) 885 MPa (128 ksi) 11-22 %
H1150 Not reported 1055 MPa (153 ksi) Not reported

The source of this data was the following article: A.J. Turner, Electron Beam Welding Thick Section Precipitation-Hardening Steel, Welding Journal, Volume 60, 1981, p 18-26.

Fracture toughness, not tensile properties, is the usual concern when welding any precipitation-hardening metal (aluminum, stainless steel, etc.). With no PWHT or only aging at ~ 480 C (900 F) toughness can be low. Aging at higher temperatures (550-620 C, 1025-1150 F) will result in welds with higher fracture toughness. I recommend that you investigate a higher temperature "stress relieve" in the range of 550-620 C (normal artificial aging temperature range) if your application is fracture-critical. A full solution treat + artificial aging PWHT will result in maximum properties.