This worked for me when I had the same problem (kinda)...
From the S4 wiki:
http://s4wiki.com/wiki/Tuning#Boost_PIDBoost PIDIf your actual boost is not meeting requested boost, you may have to increase the PID I limit between 2250 and 5000 RPM for 850 and 1000mBar:
KFLDIMX - LDR I-Regulator limit