Update https://spark.apache.org/docs/preview/ to target 4.1.0-preview4 (#651)

diff --git a/site/docs/preview b/site/docs/preview
index 5362950..97ddda4 120000
--- a/site/docs/preview
+++ b/site/docs/preview
@@ -1 +1 @@
-4.1.0-preview3
\ No newline at end of file
+4.1.0-preview4
\ No newline at end of file