check the appearance of a pattern in a string with mips assembly

Below is a program to examine the appearance of a pattern in a string. End in line 18 represents the end of the program.
• Inputs are given to array Pat (base address $s0) of type char and Str (base address $s1). Both strings have NULL characters (\0, ASCII code 0) at the end.
• Each time a pattern Pat appears in a string Str, its leading position is stored in an integer array AP (base address $s2) and the total number of appearances is output to a register $s3. For example, if the input is Pat="ABC", Str="ABCBABC", AP[0]=0, AP1=4,8>AP=.

Main:
add$s3,$zero,$zero
add$t4,$zero,$zero

LoopO:add$t0,$s0,$zero
add$t1,$s1,$t4

LoopI:lb$t2,0($t0)
beq$t2,$zero,Find
lb$t3,0($t1)
beq$t3,$zero,Fin
bne$t2,$t3,Next
addi$t0,$t0,1
addi$t1,$t1,1
j LoopI

Find:
sw$t4,0($s2)
addi$s2,$s2,4
addi$s3,$s3,1

Next:
addi$t4,$t4,$t1
j Loop0

Fin:End

This code doesn't work, if there are no characters that match array str and array pat, $t4 is much the same value even if you jump to next, so when you jump to LOOP0, add $t1, $s1, $t4 instructions point to much the same character in array str
What should I do?