4 pipelining requests sounds okay.
After a bit of reading, it may be caused by looking for search results that don't exist. I've noticed something strange on google myself these past few months where if I begin browsing through the hundreds of pages of search results, the number of results pages mysteriously shortens as I go on. There aren't as many there as it makes out there are. If you try to jump to the later ones that don't exist, then it thinks you're causing trouble.