Newbie Acol Bidding Explanation

When my friends and I started playing Acol Bridge some time ago ( 4
card majors , Weak NT (12-14 PTS) , we were told by our instructor to
bid 4-card suits "up" and 5-card suits "down" , i.e bid 4D before
showing 4S and bid 5S before 5C for instance.

Now that we have been playing a bit longer , we have forgotten the
reasoning behind this advice and one of us in particular wonders why
the 5-card bidding advice was given this way.

Can anyone remind me why this advice is given again?

B.N.
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