Next March, you'll have been learning English for four years.
Literal
Next-year [possessive-の] March [at-で] you [topic-は] English [object-を] learn-[and-て] 4-year pass-[ことになる] (it-will-be-the-case).
An even more complex duration expression: ~を学んで~たつ ('having started learning X, [time] passes') + ことになる. 経つ ('to pass / elapse') is the standard verb for time passing. The whole construction is Japanese's way of framing 'will have been X-ing for N time by Y.'