My son has been doing EPGY and there is always an end of grade test. I think that some of the grades may be combined so perhaps you will not get the end of grade for fourth but instead for 4th/5th. We are not doing OE but it should not matter.
EPGY has been great for us so far but you hit on the issue we have. My son is in 5th grade but is taking the final exam on Int Algebra tomorrow. It has only taken 8 weeks to finish the whole subject. It seems like we are too far ahead but I am not sure what else we can do at this stage. I plan to do some of the AOPS courses, such as Number Theory and Combinatorics.
I would be very interested to learn what path others have taken with math for kids that seem to have a strong ability for it?
Good Luck!
