We are allowed to assign up to 50 minutes of homework per night for our fourth graders.
Reading: My students read 20 minutes a night and fill out their "reading log" (see below).
On Tuesdays and Thursdays, students complete a "W.E.B. Log" in which they apply one of the reading comprehension strategies we practice in class to their nightly reading. By the way, W.E.B. stands for "We Enjoy Books." Right now, we're practicing what we call "Fix-Up Strategies," which is basically what you do when you come to a word or words you don't know (see below).
Students copy this into their W.E.B. Log notebooks and fill it out on Tuesdays and Thursdays. I conference with each student at least once a week to go over these and set reading goals for the next two days.
Math: Monday through Thursday, I give 4-5 math problems from the workbook that goes with our math textbook. I find that too many more questions than this is a waste of time because they make careless mistakes, and I need that extra time for either Virginia Studies or another review activity.
Virginia Studies: For each new chapter in the book, my grade level created flash cards with the state standards' language on them. Students have to draw a picture representation of what the card means and be able to explain why they chose to draw that particular picture. I usually give 2-5 of these a night, depending on how much I taught and how much other homework I've given.
Below is an example from our current unit on the documents that helped to set up our government.
All of this takes them pretty close to the 50 minutes, if not over for some of the more distractable students!
I try not to give any homework on Fridays, but it does happen from time to time. If I must give homework, I try to make it minimal.