diff --git a/labs/04_memory_debugging/README.md b/labs/04_memory_debugging/README.md
index e770cfa..80d22af 100644
--- a/labs/04_memory_debugging/README.md
+++ b/labs/04_memory_debugging/README.md
@@ -117,5 +117,12 @@ Mismatched free() / delete / delete []
 **To receive credit for this checkpoint**, discuss your answers to these questions with a TA or mentor.
 
 ## Checkpoint 4
+*remainder of the lab time*
 
-Checkpoint 4 will be available at the start of Wednesday’s lab.
+For this checkpoint, you team up with one or two other students from your lab section. Our TA or mentor will pick one problem from the practice packet for you two (or three) to work together as a group. You should not look at the posted solutions. Work as a group (e.g., pair programming!) to come up with a complete solution to the problem.
+
+Once you have completed and proofread and triple-checked your answer, launch your C++ development environment. First, focus on getting it to compile. What basic, minimal testing infrastructure is necessary to get your solution to compile?  What small bits of syntax were you missing or incorrect or sloppy? Try to avoid making these mistakes on Thursday's test!
+
+Then, write a simple test case to exercise your solution. Compile and run the code. As time follows, fix bugs in your solution and improve and expand your test cases.
+
+**To complete this checkpoint**, When about 10 minutes remain in the lab, the TA and mentors will circulate through the room to finalize checkoffs. Show them your initial solution, and explain the typos and errors you found and debugged in working through your computer tested solution. Explain your testing & debugging process.