FYI. Please refer below info how to connect 4 screens from t620 Quickspecs.
http://www.hp.com/hpinfo/newsroom/press_kits/2013/HPDiscoverBarcelona/HP_t620_quick_specs.pdf
Quad-head video option: AMD FirePro™ 2270 Multi-View Professional Graphics dual-head
graphics card (for up to four total digital monitors output) available as factory-installed
option in PLUS models only (4 GB RAM required for quad-head WES 7 configurations): Up
to 2 x 1920x1200 + 2 x 2560x1600, or with optional DMS-59 to DisplayPort adapter 4 x
2560x1600
