I think there's a big difference between capturing the whole password at once, and having to reconstruct it contextually - it may be difficult to recover the typing context. Even if the context is known 100% reliably, it will take far more than 4 logins to scrape enough information to reconstruct the password - you need overlap to match the fragments. It the user visits any other site that uses a similar scheme, it gets harder. If a a site chooses to 'salt' the password by making the user retype random characters, it gets much harder - now you need enough logins to apply statistical analysis. If the site makes the user type the characters in the wrong order, you can't reassemble the password at all - all you can do is guess the order.

To gather enough contextual information to defeat all this reliably, the keylogger would also need to be a screen scraper, internet monitor, etc. Straight away, this defeats the not-uncommon case of the inline USB keylogger.