Hi Nickymomma,
 
The first place I would recommend for you to visit is Education.com's Reading Comprehension page:
http://www.education.com/topic/reading-comprehension/
 
One article in particular that you may find helpful is this one on comprehension strategies:
http://www.education.com/reference/article/comprehension-strategies/
 
You might also want to read up on "How a 3rd Grader Thinks:
http://www.education.com/magazine/article/How_child_thinks_third_graders/
 
Or likewise, "How a 4th Grader Thinks":
http://www.education.com/magazine/article/How_child_thinks_fourth_graders/
 
Both articles reference comprehension ability at these grade levels.
 
We also have some reading activities that help make the learning process fun: http://www.education.com/activity/all-grades/reading/
 
I hope all the above helps your third and fourth grader(s) with their reading comprehension!