All I know if that it can cause insomnia in some people because of the T3. So that might be why it says not to take it after 3 pm. I'm on such a low dose right now, that I don't think I could 'split' mine. So I just take it in the morning sublingual. So far I don't feel as wacked out on it as I did synthroid. But then again, I am on a low dose..... and will slowly raise it. At least I don't feel worse this time,... just not a ton better. But it's early!
As long as you aren't having any insomnia problems like Anree mentioned, 4pm should be fine. I usually try to take mine at 8am, and between 2-3pm. Sometimes I'd get busy at work, and totally forget to take the 2nd half until 7pm. I would hit a slump at 3pm and be really tired and wonder why. Then later I'd remember it's cause I forgot my other dose!