Ex-BH bought calves to bucket rear for years. Usually Hereford crosses out of Friesians. He'd give them 2-3L three times a day to one month old, then get them onto two feeds a day.
Your Dexter is quite a bit smaller, of course, but 2L a day sounds very little to me, plus at only 4 days old I think he needs more feeds, at least three if not 4 in a 24 hour period.
Personally I would add a third feed, and probably up the daily total to at least 3L, if not 4.