.check is a request DSL element, just like get, headers, not a structure DSL one. You’re not using it at the right place.
Wrong:
.exec(http("request_4")
.get("""/cas/js/cas.js;jsessionid=S44VxB-WMJcuovw0HKFG6ULW.undefined""")
.headers(headers_4))
.pause(23)
.check(css("input[name='lt']", "value").saveAs("lt")))
Right:
.exec(http("request_4")
.get("""/cas/js/cas.js;jsessionid=S44VxB-WMJcuovw0HKFG6ULW.undefined""")
.headers(headers_4)
.check(css("input[name='lt']", "value").saveAs("lt")))
.pause(23)