Bump TF to 2.4.0rc4 (#1213)

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Author: yongtang

tensorflow_io/core/python/ops/version_ops.py

@@ -14,5 +14,5 @@
 # ==============================================================================
 """version_ops"""
 
-package = "tensorflow>=2.4.0rc3,<2.5.0"
+package = "tensorflow>=2.4.0rc4,<2.5.0"
 version = "0.17.0"