Picked up Jamaal Williams this morning but the website says the transaction won't happen until week 5. How do I get him in my lineup today?

You don't. Apparently you used a player who was in your lineup this week to pick him up. You can't do that and have him count for the same week.

I have no idea.  In most leagues, the common waiver setting would have had him as a FA yesterday or Sunday.  But some leagues lock everything after Thursday or whatever.  Look at your league waiver settings.

PS - It could also be what Leg said, but they usually send an alert that the transaction is ineligible.

You probably don't.

Most likely you were trying to drop someone that already played in Week 4, so of course you can't have an extra guy for the week.  

My league is run by liberals who say everything must be equal.  If someone makes a waiver pickup that could really help, the commissioner cancels the waiver. But like with most liberal systems the rules do not apply to the commissioner.