Allison,
You've gotten some wonderful counsel from Tansy and Water2wine. I also would err on the side of making sure to give your kids enough time to grow into being independent readers.
So, with some of the reading challenges you mentioned with your kiddos, since you're planning to use "Bigger..." for them, I would look at the Emerging Reader Pack to see if that might be the best fit for your third grader. Here is the link:
http://www.heartofdakota.com/emerging-reader.php
The schedule for the Emerging Reader Pack is in the Appendix of the "Bigger Hearts..." guide. It tells you which pages to have the child read aloud to you each day and 3 questions to ask about the reading.
For your 4th grader, I would look at "Drawn into the Heart..." Level 2/3. I would decide between the two book packs (level 2 or level 3) to see which suits your kiddo best. Here is the link to those packs (click on each Level to see book descriptions):
http://www.heartofdakota.com/drawn-into-optional.php
With "Drawn into the Heart...", it is also important not to jump up too quickly to the next level. You'll want the foundation provided in Level 2/3 and the time for them to grow into being more fluent readers before Level 4/5.
The Storytime Part of the plans also has a daily literature study component, so your kiddos will both be getting literature study there as well with your read-alouds. I wrote it this way to still provide literature study until most kiddos will have had enough time to grow into "Drawn into the Heart...".
Hope that helps!
Blessings,
Carrie