There was quite a bit of nute lock-up to get a 4ec. Usually at this stage I would be running an ec of 2.0-2.4. If everything is going good I have managed 3.0 ec. After my weekly res changes I will only add water and only adjust ph. This way my ec either stays the same or drops. After the change I usually see a decrease in ec and ph goes up so I try to start a new resevoir at 5.8 and work towards 6.2 to make sure the plants recieve all nutrients. When I am near harvest I only flush for a week, but I drain the resevoir every day and add fresh new water till the ec is almost nil. Its a little more work, but you can flush without doing that. Just change the res at least once during the flush to remove the initial flush salts.