The remainder of the second order from 534.174 to 534.193 were the IV series. With later orders the fourth series would occupy the serial numbers .174 and 445 and was therefore the most numerous of the types. The most important change to the earlier series was the enclosed cockpit. The IV series also enjoyed a metal light alloy Letov (Hochfeld) Hd-43 propeller. The various refinements allowed the IV series to have an increased speed of over 200 mph, and this placed it on par with the best of its contemporary biplanes. A common alteration to the IV series was the replacement of the tailskid with a tailwheel. Operational experiences had shown that the spat on the main landing gear could become clogged with mud on grassy airfields and cause takeoff and landing problems. The spats were then often removed.