tweets_filter.user.js

// ==UserScript==
// @name           tweets_filter
// @namespace      http://www.relucks.org/
// @include        http://twitter.com/
// ==/UserScript==

var re = /(4sq.com)/
// var re = /(4sq.com|#example)/

filter(document)
document.addEventListener("DOMNodeInserted", function(e){
    if (e.target.tagName) {
        filter(e.target)
    }
}, false)

function filter(node) {
    Array.forEach(node.querySelectorAll('span.entry-content'), function(i) {
        if (i.innerHTML.match(re)) {
            var li = i.parentNode.parentNode
            li.style.opacity = 0.2
            // li.style.display = 'none'
        }
    })
}