We will be using Singapore Math 3A and 3B soon and we have the rest of our books already. As a filler,we are using Spectrum Math 5,(it was about $2 through Amazon), and she's able to do the work without any difficulty so far.
It's not really a big jump - Singapore Math seems to be ahead of other math programs anyway.
My daughter started homeschooling at three, so she's on grade 3/4 in math. She's multiplying and doing fractions and division, AlhamduLILLAH. I want to be consistent so I am sticking with the Singapore Math and we both like it very much.
I am also using Spectrum Language Arts Grade 4, Spectrum Reading Grade 4, and Spectrum Writing Grade 4.
She is doing well so far and she's getting plenty of practice with writing. What I've seen so far is that she can read and write, but needs to develop her imagination. I think that she has gotten so used to rules, (like phonics and math rules and spelling rules, etc) that she hasn't spent enough time being creative. I guess we should work on that, lol.
She's also reading book number 44 out of her 50 required chapter books. She has until August but I'm hoping that she does it sooner than later.