<select onchange="mmm(this.value)">
<option value="11">1111</option>
<option value="22">2222</option>
</select>
<input type="checkbox" value="4" name = "btn4" id="btn4">
<input type="checkbox" value="5" name = "btn5" id="btn5">
function mmm(a){
if(a==11){
document.getElementById("btn4").checked=true
document.getElementById("btn5").checked=false
}
if(a==22){
document.getElementById("btn5").checked=true
document.getElementById("btn4").checked=false
}
}
<option value="11">1111</option>
<option value="22">2222</option>
</select>
<input type="checkbox" value="4" name = "btn4" id="btn4">
<input type="checkbox" value="5" name = "btn5" id="btn5">
function mmm(a){
if(a==11){
document.getElementById("btn4").checked=true
document.getElementById("btn5").checked=false
}
if(a==22){
document.getElementById("btn5").checked=true
document.getElementById("btn4").checked=false
}
}
<select onchange="mmm(this.value)">
<option value="11">1111</option>
<option value="22">2222</option>
</select>
<input type="checkbox" value="4" name = "btn4" id="btn4">
<input type="checkbox" value="5" name = "btn5" id="btn5"><script>
function mmm(a){
if(a==11){
document.getElementById("btn4").checked=true
document.getElementById("btn5").checked=false
}
if(a==22){
document.getElementById("btn5").checked=true
document.getElementById("btn4").checked=false
}
}
</script>