Thinking through it, I'm pretty sure 5 is the correct batter, no action to be taken.
The appeal should have been when 4 got on base. Then 3 would be out. Since that wasn't appealed before the next batter received a pitch, he remains on base. Then there could have been an appeal that 3 was in the wrong position after he completed his at bat because 5 should have been up. However, by not appealing before a pitch to 5 - that opportunity is lost.
As soon as a pitch is thrown - all previous out of order batters become legalized. The only batters you really need to worry about are the one just up and the one currently up. The current batter is never the one you call out - you call out who didn't bat in the proper spot.
In your scenario, 3 could have been called out for not batting in his turn, but only after 4 completed his at bat. At that point 4 should have been up again, with his previous hit nullified. He should have then been followed by 5. However 3 batted in his spot, so in theory 5 could be called out if there was an appeal.
Since there was no timely appeal, you have 3 as now being legalized and should be followed by 4. The monkey wrench - 4 is on base. In that case, he is skipped in order - and the next batter comes to the plate. In your case that's 5.