Right now we're working on adding decimals in our fourth grade math program. My children understand this concept of tenths and hundredths because they've worked with small amounts of money, but if a number is written .3 rather than .30, they'll sometimes get confused. I'd been making up random numbers to pop into worksheets until I got an email alerting me to the dadsworksheets.com website. (Yes, it's free!) It didn't take much digging for me to find the decimal addition and subtraction worksheets already made up for me. Yay!
The only caveat I would pop in here is that "fourth grade math" can vary by curriculum, so I can't just pop onto the "fourth grade math" tab on the sidebar and get started that way. We aren't using any negative numbers whatsoever in fourth grade, but we're working with decimals, fractions and geometric problems extensively.