If it's VERY extended, yes, it can. The kidneys don't flush properly at low salinity, which can cause toxins to build up over the long term. From personal experience, even 2 months in hypo is not long enough to cause problems.
EAGLES made a good point, it's not 4 weeks start to finish. You need to go 4 weeks from the last visible sign of ich. It's best if you QT in a bare tank that you syphon regularly. A bare tank is always preferable over one with rocks and substrate. It's harder for ich to settle, encyst, and reproduce in a bare tank.