User will not be allowed to enter more than 4 digits and onKeyPress allowed only number/decimal

example

<input type="number" pattern="/^-?\d+\.?\d*$/" onKeyPress="if(this.value.length==4) return false;" /> 

Allowed  Only Number and Set Max Length
Note : If  Decimal Number Allowed  Change Perimeter true 
onkeypress = 'return isNumberKey(event, true ,this.value)'

second example

<input type='number' class='qty' name='' value='' min='0' max='99' onkeypress='return isNumberKey(event, false, this.value)' />

function

function isNumberKey(evt, decimal , value){
        if(value.length==2){
            return false;
        }
        var charCode = (evt.which) ? evt.which : evt.keyCode
        if (decimal == true && charCode == 46)
            return true;
        if (charCode > 31 && ((charCode < 48 || charCode > 57) || (charCode >= 96 && charCode <= 105)))
            return false;
        return true;
}