On the scale of "how many" is this a common (daily, for example) error? Is it consistent? Does it come only from a particular ip or range of ips?
Malformed requests are fairly common on the web and if the site is returning a 4x error, then, in most cases, it's not that big a problem. If it is hundreds/thousands of times daily then, yes, one needs to look at the site to see if there's something wrong.