In my humble (?) opinion, any of the Schubert impromptus -- and, for that matter, most music -- should be learned first, and brought up to near performance technical accuracy and speed, including a clear distinction between legato, detached, and stacatto notes, without the use of the sustain pedal. I do not always practice what I preach...
I might note that there are occasions where certain passages are simply not possible without a very discreet use of the sustain pedal.
Once that is done, then as you start to add interpretation to technique you can also begin to add sustain pedal where it seems to belong. Sometimes this will be to allow a series of notes to meld (Op. 90 #3, parts (but sparingly) of Op. 90 #4). Sometimes it will be to change the tone colour of a particular chord or note (for example -- in Op. 90 #4 I depress the sustain pedal just before each of the two closing chords, but hold it and my hands only just as long as they are notated). Etc...