求助液压高手

有套液压系统图有点不明白，请高手指点迷津，谢谢！有能做的可以给我留言，我的QQ125548126。

1是平衡阀，也就是单向顺序阀
2是2位2通换向阀  不过Limit switch的意思极限开关，应该是行程阀，类似于电气的行程开关
3是多路换向阀
4是从阀的职能符号上看不出来，从英文上Single over centre valve看，也翻译不出来，不过搜索后确实有该英文定义。

4./         Single over centre valve
经查，4也是单向顺序阀，即平衡阀。

Overcenter valves resemble counterbalance valves in that their purpose is to maintain a set pressure opposite a load, to keep it from free-falling. The main difference is that an overcenter valve uses a pilot signal, usually from the inlet of the actuator, to assist in opening the spool. This pilot assist makes the overcenter valve more efficient, and reduces the horsepower requirement and heat generation within the system.

Overcenter valves resemble counterbalance valves in that their purpose is to maintain a set pressure opposite a load, to keep it from free-falling. The main difference is that an overcenter valve uses a pilot signal, usually from the inlet of the actuator, to assist in opening the spool. This pilot assist makes the overcenter valve more efficient, and reduces the horsepower requirement and heat generation within the system.

4  是一个阀体的组合 没有画出来，从原理上看应该是  双液控单向阀  望指教

谢谢各位高手指教！