My 4.5 month old baby has difficulty sleeping at 6pm each night, what can it be?

She's a little old to be starting with colic but her symptoms really do sound like it. Have you tried gripe water? It works wonders. I would also suggest laying her down for a nice full body warm oil or warm lotion mage just a few minutes before she usually starts up and continue for however long but at least 10-15 minutes, mage has a long lasting soothing and calming effect on babies.