Do not confuse FST4 with FT4, which has a very different purpose!
FST4 is is designed for making 2-way QSOs on the LF and MF bands.
Operation with FST4 is similar to that with other _WSJT-X_ modes: most
on-screen controls, auto-sequencing, and other features behave in
familiar ways.  However, operating conventions on the 2200 and 630 m
bands have made some additional user controls desirable.  Spin boxes
labeled *F Low* and *F High* set lower and upper frequency limits used
by the FST4 decoder, and these limits are marked by dark green
angle-bracket symbols *< >* on the Wide Graph frequency scale:

image::FST4_Decoding_Limits.png[align="center"]

{empty} +

image::FST4_center.png[align="center"]

It's best to keep the decoding range fairly small, since QRM and
transmissions in other modes or sequence lengths will slow down the
decoding process (and of course will be undecodable).  By checking 
*Single decode* on the the *File | Settings | General* tab, you can
further limit the decoding range to the setting of *F Tol* on
either side of *Rx Freq*.

