When you say flock tag, do you mean they don't have individual id numbers? (ie they just have the flock number, and no other number)
If they are 3 or 4 years old then they only need one ear tag, but it needs to have the flock number and an individual ID number. It doesn't need to be EID.
So, if they have the two numbers on their tags they are ok. If they only have the flock number on they are not ok, and you will need to get a set of red replacements tags which will have your flock number on and individual id numbers, and you will need to record these tags in the "replacement tags" section of you flock book. You may as well get EID tag pairs (one EID and one non-EID) as you will have some left over and the likelihood is you will use those on newer sheep which will need EID.