BTW, now that I have the ear (eyes) of a 4 volume user, have you used Christian Prayer before? If so, then may I ask - if the MP and EP are identical, then is it true the 4 volume has a fuller Day prayer + OOR, and there’s the difference? If this is correct, going from one to 4 volumes, man, that’s a heckuva lot of OOR and day prayer. Or am I missing something?
Each of the four volumes covers a season of the liturgical year: Advent/Christmas (I), Lent/Easter (II), first half of Ordinary Time (III), and second half of Ordinary Time (IV). That way you only have to carry around one book at a time for the entire season.
That being said, it is thus necessary for certain parts (e.g., the Four-Week Psalter, Night Prayer, other parts commonly used throughout the year) to be repeated in each of the four volumes. So the 4-volume does not really have four times as much “new material”; there is a significant amount of overlap.
The main difference, though, is in the Office of Readings. Christian Prayer has a small sampling of readings. 4-volume LOTH has unique First and Second Readings for every day of the year. The First Reading is a longer reading from Scripture. During Advent/Christmas/Lent/Easter, they are seasonally appropriate and come from various books of the Bible. During Ordinary Time, they systematically cover most books of the Bible over a period of time, not exhaustively, but hitting all the high points (for example, several days of last week covered the Book of Jeremiah). Along with daily Mass readings, this is my major diet of Scripture each day. The Second Reading has a substantial reading each day, either from one of the Church Fathers, or from/about the saint whose feast is celebrated that day, or from a Church document (e.g., one of the documents of Vatican II). I find these incredibly enriching for knowledge of the faith and as a starting point for meditation. Office of Readings is my favorite part of LOTH, and I would not want to give up the 4-volume for that reason alone.
I’m not sure about the Daytime Prayer part; I don’t remember how much of that is in Christian Prayer. In the 4-volume, there are three “hours” of Daytime Prayer for each day (Midmorning, Midday, and Midafternoon), so if that is not included in Christian Prayer it would add a considerable amount to the Four-Week Psalter.
You can buy the volumes one at a time (sometimes used online) if it is too expensive to do all at once. Or you may want to borrow a friend’s for a half hour for a few days to see whether you like it before you make the investment.
I hope this made sense!