Well, that depends on what type of LLLL.com's you have. Generally, if you have a name like XTFQ.com, and make a site about Race Cars, Electronics, ect... (Basically whatever you choose) I think the average user would have a better time remembering a name that actually made sense rather than 4 random letters. Personally, I think that 4 letter names are only worth developing if they are actually an acronym or are specific towards the type of site you are developing. Not to mention they suck for SEO purposes....... Brandable names, whether they are 5, 6, 7 letters ect, are generally good for blogs, social sites, random content sites, media, ect. If you are looking to promote products or a niche service or area of expertise, you would have better luck using a name that is longer but actually has relevance to the site.