You need jack studs unless the header is built into the floor system above. (Part of the rim joist, extending out past the opening to sufficient bearing on each end). I recommend temporary support. There may be a point (concentrated) load above from a window, etc. that a temp. wall would carry while doing the work. The rim joist may be joined or pieced in over this 4' wide opening. Really hard to see without being there..... for all the variables that could be.
As per minimum building code, you need 2 jack studs on each end with a header carrying your load and spanning 2'11" if house is 36'. Match your house width, load of roof, ceiling, and two floors for the proper sized header and jacks required: http://www.burlington.org/Building08/Spans.pdf
Check with your
local Building Department for verification. They may also have requirements about the window location; in a shear panel required for wall bracing, size of window percentage of floor space required for light or/and ventilation, egress size - if required for bedroom, etc. eg- http://illowaicc.org/uploadedFiles/I...20Openings.pdf
Be safe, Gary
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!