4% is annual not monthly interest. On a single account with say 7k in you are making about £20 interest.
With x2 accounts of 5k you will be making about £30 a month interest.
Halifax reward accounts are similar and you can seemingly keep £1 in each of the 3 accounts then literally pass £1k through them in 10mins then back out, £5 from each account each month.
/profit!
