Singapore has placement tests, and they strongly recommend using them.

For my DD, who is currently in 4th, we're doing some enrichment this summer. I got the 3B and 4A textbooks, and the Level 3 Challenging Word Problems. If she flies through them, we can always order the next set up. (I think she could have passed a good chunk of the 3B material, but she hasn't had significant exposure to weights and measures. Since we're aiming for enrichment, not advancement, that's a good topic to enrich.)

Singapore also has some speed drill / mental math materials that I strongly considered. Mental math is not my DD's strong suit.