Offshore wind farms worldwide are moving further away from shore in order to capture more favourable wind conditions. However, this creates additional challenges regarding their Operations & Maintenance (O&M). O&M costs contribute significantly (20-30%) to the Levelised Cost of Energy (LCOE) over the lifetime of an offshore wind farm. One of the main reasons is the relatively low accessibility to the wind farm, which increases the potential downtime and therefore lost energy production, especially for far-offshore wind farms. This has driven the research into innovative access systems which promise average annual accessibility of 90%.In this fourth edition, an updated overview is presented of commercially available and demonstrated access systems for offshore wind farms. Three categories of access system are identified, based on the point of access towards the wind turbine: i) access to the boat landing, ii) access to the platform of the transition piece, and iii) access to the helicopter hoisting platform on top of the nacelle. Besides the conventional method of access to the boat landing through Crew Transfer Vessels (CTVs), motion compensated gangways mounted on the deck of Walk-to-Work (W2W) Vessels or dedicated Service Operation Vessels (SOVs) have entered the market during the last decade, which has moved large parts of the maintenance base offshore. The analysis in this report shows that the growth of this market is in alignment with the needs for safer and more efficient transfer of technicians and cargo towards the wind turbine. In addition, while the relatively close-to-shore locations of current offshore wind farms has driven the extensive use of CTVs, more SOVs are being deployed as wind farms move further from shore and wind turbines become larger in size, and new designs for motion compensated gangways and cranes (for SOVs) are being introduced. Furthermore, the following trends are discovered: The motion compensated gangway for CTVs are still not well adopted by the industry.Within the category of CTVs, the market share of SWATH vessels (a type of CTV with improved seakeeping performance) is increasing; In addition to the wind farm installation and (short-term) maintenance campaign, a rising number of dedicated SOVs are entering the offshore wind industry for (long-term contract) daily operation and maintenance (O&M); New forms of propulsion such as batteries, hydrogen and hybrids are prototyped to reduce emissions, especially for CTVs. It is important to model offshore wind farm Installation / O&M activities to fully assess the impacts of a particular access solution. By doing so, design drivers can be identified, business cases of new systems can be established and optimal decisions can be made both for defining the Installation / O&M strategy and choosing between different access solutions. Wind Group of TNO has been constantly developing software tools (e.g. TNO Install, TNO O&M Planner and TNO Despatch) to make such analyses, and will constantly upgrade these tools to improve the fidelity and user-friendliness.