How to quickly score 5,000,000 points in Super Mario Advance 4: Super Mario Bros. 3?
The best way to do this, as far as I know, is by repeating the following:
Enter World 7-3. Grab the Starman from the block, then defeat the first four Koopa Troopas. Jump the gap without grabbing an additional Starman. Before the first Starman has run out, defeat the next four Koopa Troopas and Paratroopas (two of each). Run back if you miss one, there's a little time to spare. Return to map.
With a little practice, you can do this repeatedly for 17,500 points in less than 10 in-game seconds. This will get you 5,000,000 points in an hour at most.
Another method suggested on the internet is playing World 7-Fortress 1 and repeatedly using the P Switch to pick up a bunch of coins. However, I found this method significantly slower and would not recommend using it.