MIPS error "operand is of incorrect type"

Question

I am trying to convert C code into MIPS assembly code, however I am getting an error:

"$t4": operand is of incorrect type

I could not find the error. Here is my C code and assembly code:

int A[4];
int i;
int diff;
for(i=0; i<3; i++){
   diff = A[i+1] - A[i];
   if (diff > 0)
     A[i] = 5*A[i];
   else
     A[i+1] = -5*A[i];
}

MIPS code:

.data
    intArr: .word 2, 4, 6, 8
.text
    main:
        la $s3, intArr      # address of start of intArr $t1 
        li $s0, 0       # load loop iteration variable (i) to $s0
        li $t2,3        # constant $t2 = 3

    Loop:
         bge $s0,$t2 end    # end program when 3 iterations happened
         # diff = intArr[i+1] - intArr[i]
         lw $t1,intArr($s3)         # $t1=intArr[i]
         addi $t3, $s3,4    # intArr[i + 1]
         lw $t4,intArr($t3)     # $t4 = intArr[i + 1]
         sub $t5, $t4,$t1       # diff = $t4 - $t1
         bgt $t5,0,else     #check if diff($t5) > 0
         addi $t6,$zero,$t4
         addi $t6,$t6,$t4
         addi $t6,$t6,$t4
         addi $t6,$t6,$t4
         sw $t6,intArr($s3) 

         else:
            subi $t6,$t4,$zero
            subi $t6,$t4,$t4
            subi $t6,$t4,$t4
            subi $t6,$t4,$t4
            subi $t6,$t4,$t4
            subi $t6,$t4,$t4
            subi $t6,$t4,$t4
            sw $t6,intArr($s3) 



        addi $s3, $s3,4     #next element A[i+1]
        addi    $s0, $s0, 1     #Add immediate value 1 to i (i++)
        j Loop              #Jump back to the top to loop again

    end:
        li $v0,10
        syscall