10 DIM A$(4):DIM PN(3)
20 MAX=100:DLY=5:REM adjust to taste
30 CW=0:REM CW=1 for CCW rotation. Set to 1 or 0 depending on which direction to start in
40 FOR X=0 TO 3:READ PN(X):NEXT X:REM read which pin numbers to use into array PN()
50 DATA 15,16,17,18: REM PIN numbers used to drive stepper motor
60 FOR X=0 TO 3:SETPIN PN(X),8:NEXT X:REM set pins in PN() array to digital output
70 DATA "0111","1011","1101","1110".1111":REM codes to drive pins on or off
80 FOR X=0 TO 4:READ A$(X):NEXT X:REM read codes into string array A$()
90 DO:REM main loop
100 IF CW=0 THEN 110 ELSE 130:REM CW=0 for clockwise, non-zero for CCW rotation
110 FOR STP=0 TO 4:REM there are 4 steps to do; refer line 70
120 GOTO 140
130 FOR STP=4 TO 0 STEP -1:REM as for line 110 but reverse direction
140 FOR Y=0 TO 3
150 PIN(PN(Y))=VAL(MID$(A$(STP),Y+1,1)):REM extract binary 0 or 1 from a$() array element
160 NEXT Y
170 PAUSE DLY: REM increase PAUSE value to SLOW rotation.
180 I$=INKEY$:IF I$="" THEN 230:REM read keypress
190 IF I$="Q" OR I$="q" THEN PRINT"STOPPED":END
200 IF I$="R" OR I$="r" THEN CW=NOT(CW):PRINT"REVERSE"
210 IF I$="F" OR I$="f" THEN PRINT"FASTER":DLY=DLY - 1: IF DLY<0 THEN DLY=0
220 IF I$="S" OR I$="s" THEN PRINT"SLOWER":DLY=DLY+1: IF DLY>MAX THEN DLY=MAX
230 NEXT STP:REM next step loop
240 LOOP:REM end of main loop

As you speed up a stepper, the current drops off and so does the torque and eventually the motor has so little torque that it just vibrates. To get over this, people use constant current sources along with higher supply voltages so that the current stays high even when the stepper speeds up.