Mips programming jump problems - windows

Question

Supposed to count number of R, I, and J instructions in the code starting from the first line in main. I have updated the code to all suggestions but results do not seem correct still. :(
EDIT: added those syntax changes, the variables seem to be working better, but the loop doesn't seem to be properly running through itself at all. Any noticeable problems anyone sees?
EDIT2: How do i change the loop to make $t1 rotate through 1 instruction set at a time? THAT is my key problem right now other than the silly syntax from a new mips user.
EDIT3: I have updated the code to all suggestions but results do not seem correct still. :(
.text
Main:
la $t1, 0x400000
li $t5, 0
li $t3, 2
li $t4, 3
li $t2, 0
la $s0, 0x400000
loop:
lw $t1, 0($s0)
addi $s0, 4
addi $t5, 1
beq $t5, 20 exit
srl $s4, $t1, 26
beq $s4, $t2 R
beq $s4, $t3 J
beq $s4, $t4 J
addi $s3, 1
j loop
R:
addi $s1, 1
j loop
J:
addi $s2, 1
j loop
exit:
sw $s1, RType
sw $s2, JType
sw $s3, IType
jr $ra

Answer 1

I see a set of problems:
1) Initialize the registers (don't assume register initial value is zero). For example, addi $t5, 1 requieres a previous add $t5, $zero, $zero (or li to the value you want).
2) the beq jump instruction requires two registers. For example, you have to change beq $s4, 3 J by:
li $t6, 3
beq $s4, $t6, J

Answer 2

In addition to Miguel's points I'd like to add that you're using the instruction set in a strange manner. Perhaps your assembler is capable of figuring out what you meant, but it's still makes the code confusing to read.
lw $t3, 2
lw $t4, 3
These should be li. The lw instruction is used for loading words from memory.
la $t0, 4($t1)
This should be lw, and unless you only want to check the instructions starting from address 0x400004 you should drop the offset.
I also can't seem to find anywhere where you increment $t1, so you'll be using the same address for every iteration of the loop.