You can't set up timed messages with group policy, but you can execute scripts. You can only specify logon or logoff scripts though, so unless you want to specify a logon script that sleeps until 4:30 and then pops up a message, you're probably out of luck. That might work, but if your user logs in at 8am there's a lot that could happen that could prevent that script from actually executing.
If they are on the same PCs every day, you could schedule a script to run at 4:30 that pops up a messagebox. That would be pretty easy. I use a similar scheduled script to reboot certain PCs in the middle of the night.
If it is the same group of people every day, you might be able to schedule a single script on your server to run that does a NET SEND to those specific users.