We've done basically what Ann mentioned (combined Singapore with Miquon) and since we're a few years further down the road, I just wanted to say that it's a combination that we've been very happy with. Basically we started with Miquon at age 6 or so (though one or two of my kids used the Singapore EB books prior to that for 'math play' like their older sibling(s)), began overlapping with Singapore by the time they got into the 2nd-grade level stuff, and then moved over to Singapore completely by the 3rd-grade level. I've got one kid in Singapore's secondary program, one in Singapore 3 and one in Singapore 4 and they've all got terrific math skills.
The combination of a great manipulative-based foundation in numbers and operations thanks to Miquon and clear, logical, creative and challenging work on mathematical thinking and computational algorithms in Singapore has worked very well for us. Both programs all lend themselves well to being worked through in leaps and spurts by bright, math-intuitive children.
Edited to add that 'turning four in March' is much younger than I would choose to start a math curriculum, and certainly it wouldn't work with Miquon unless you have a very unique child. One of my kids did, at her insistence, start Miquon at 4.5, but she was already fairly capable with basic addition, subtraction and multiplication. Even so, she moved quite slowly compared to my other kids who started at 6.