HBase HDFS zookeeper

HBase HDFS zookeeper

Now I am learning about HBase. I set up my HBase Cluster and Hadoop
Cluster like this:
server1: Namenode HMaster
server2: datanode1 RegionServer1 HQuorumPeer
Server3: datanode2 RegionServer2 HQuorumPeer
Server4: datanode3 RegionServer3 HQuorumPeer
I have several question about HBase cluster:
1: All RegionServers must be in the Hadoop Cluster so it can use HDFS to
store
data, even though it will store data into local file system, right?
2: What does RegionServer do? Does the HMaster give the job to all
RegionServeres
and let them running parallel, like tasktracker in datanode?
3: What does zookeeper do? Do I need to setup zookeeper in all RegionServers
nodes and the master node?
4: It is related to #3. I know HBase uses zookeeper to recovery once
regionServer
is down. How does it specific work?