Zone 4 is situated in the lower estuary between Rowella and George Town. The estuary here is wide and deep with Middle Arm and West Arm joining the estuary on the western shore. The benthic habitat is mainly silt in the southern half of the zone with cobble and reef predominating in the northern section from Deceitful Cove to George Town (Lucieer et al., 2009).
There are a number of introduced pest species found in this area and the greater Tamar estuary including the Pacific oyster, Asian bag mussel, European green crab and rice grass (Aquenal & DEPHA, 2008).
The main population centres are Beauty Point on the western shore and George Town on the eastern shore. Bell Bay is the most industrialised section along the estuary supporting mining, metal processing, timber and shipping industries.The area has a history of elevated metal concentrations in sediments from historic practices of both mining and metal processing. Zone 4 is also widely used for recreational activities such as skiing, swimming, fishing and boating.
The sample salinity ranges from estuarine to full marine at 3.0-35.0 ppt with a sample depth range of 4-42 metres). There is a strong marine influence, and less stratification of the water column compared to Zones 1-3. Surface water quality may still influenced by freshwater flows.