Here's something to get you started:
SELECT ID, LastOrderDate, Total, Segment
FROM SegmentA
UNION ALL
SELECT ID, LastOrderDate, Total, Segment
FROM SegmentB
WHERE ID NOT IN (SELECT ID FROM SegmentA)
UNION ALL
SELECT ID, LastOrderDate, Total, Segment
FROM SegmentC
WHERE ID NOT IN (SELECT ID FROM SegmentA)
AND ID NOT IN (SELECT ID FROM SegmentB)
UNION ALL
SELECT ID, LastOrderDate, Total, Segment
FROM SegmentD
WHERE ID NOT IN (SELECT ID FROM SegmentA)
AND ID NOT IN (SELECT ID FROM SegmentB)
AND ID NOT IN (SELECT ID FROM SegmentC)
A very simplistic answer, more information is needed if you want to optimize this.