Premiere Elements does not let you go lower than a still image default duration of less than 1 frame. In a 30 frames per second set up, this equates to 0.033 seconds; in a 25 frames per second set up to 0.040 seconds.
You can change the still image duration
a. BEFORE you import media...Edit Menu/Preferences/General...there the default is expressed in frames, 150, which equates to 5 seconds (30 frames per second system) or 6 seconds (25 frames per second system). This setting sets the still image duration for all stills imported AFTER it is set. Once set it remain in effect until you change it.
also another opportunity for still image default duration
b. After the media is imported, you could right click the still image, select Time Stretch and set the still image duration (but here again, not less than 1 frame). The Duration readout in the Time Stretch dialog is represented 00;00;00;00 (hours;minutes;seconds;frames). So, a 1 frame setting would look like 00;00;00;01.
Add On...if you have a 4 seconds PNG file, what duration are you aiming for? That "1" that you are seeing in preferences is in units of "frames" and is 1 frame (0.033 seconds NTSC or 0.040 PAL). See above.