Prueba esto:
Código PHP:
<html>
<head>
<title>Untitled</title>
</head>
<body>
<input type="text"
name="valor"
size="5"
maxlength="5" onBlur='if(isNaN(this.value)||this.value>99){this.value="";this.focus()}'>
<input type="button"
value="pincha"
onclick="if(valor.value){a=Math.round(valor.value*4)/4;alert(a)}"
>
</body>
</html>