I would recommend balancers as I have personal experience of AGM getting unbalanced, and I believe this should be avoided. Especially since the balancer is cheap.
Victron suggest their balancer can be used for up to 3x 200Ah strings (pairs), so I would suggest you use two balancers. You should connect the midpoints of two strings together and use the balancer on two strings. Do this twice for your 4 strings.
I will be doing something similar with my 6 strings if my replacement batteries end up being AGM.