using System;
using Iesi.Collections;

class One {
    private Guid _ID;
    private ISet _Twos;
    private ISet _Threes;

    public Guid ID {
        get { return _ID; }
        set { _ID = value; }
    }

    public One() {
        _Twos = new HashedSet();
        _Threes = new HashedSet();
    }

    public ISet Twos {
        get { return _Twos; }
        set { _Twos = value; }
    }

    public ISet Threes {
        get { return _Threes; }
        set { _Threes = value; }
    }
}

class Two {
    private Guid _ID;
    private One _One;
    private ISet _Fours;

    public Guid ID {
        get { return _ID; }
        set { _ID = value; }
    }

    public Two() {
        _Fours = new HashedSet();
    }

    public One One {
        get { return _One; }
        set { _One = value; }
    }

    public ISet Fours {
        get { return _Fours; }
        set { _Fours = value; }
    }
}

class Three {
    private Guid _ID;
    private One _One;
    private ISet _Fours;

    public Guid ID {
        get { return _ID; }
        set { _ID = value; }
    }

    public Three() {
        _Fours = new HashedSet();
    }

    public One One {
        get { return _One; }
        set { _One = value; }
    }

    public ISet Fours {
        get { return _Fours; }
        set { _Fours = value; }
    }
}

class Four {
    private Guid _ID;
    private Two _Two;
    private Three _Three;

    public Guid ID {
        get { return _ID; }
        set { _ID = value; }
    }

    public Two Two {
        get { return _Two; }
        set { _Two = value; }
    }

    public Three Three {
        get { return _Three; }
        set { _Three = value; }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping
   xmlns="urn:nhibernate-mapping-2.0"
   assembly="Test"
>
  <class
    name="One"
    dynamic-update="true"
  >
    <id name="ID">
      <generator class="NHibernate.Id.GuidCombGenerator, NHibernate" />
    </id>
    <set name="Twos" inverse="true" cascade="all" lazy="true" >
      <key column="OneID" />
      <one-to-many class="Two" />
    </set>
    <set name="Threes" inverse="true" cascade="all" lazy="true" >
      <key column="OneID" />
      <one-to-many class="Three" />
    </set>
  </class>

  <class
    name="Two"
    dynamic-update="true"
  >
    <id name="ID">
      <generator class="NHibernate.Id.GuidCombGenerator, NHibernate" />
    </id>
    <many-to-one name="One" column="OneID" not-null="true" />
    <set name="Fours" inverse="true" cascade="all" lazy="true" >
      <key column="TwoID" />
      <one-to-many class="Four" />
    </set>
  </class>

  <class
    name="Three"
    dynamic-update="true"
  >
    <id name="ID">
      <generator class="NHibernate.Id.GuidCombGenerator, NHibernate" />
    </id>
    <many-to-one name="One" column="OneID" not-null="true" />
    <set name="Fours" inverse="true" cascade="all" lazy="true" >
      <key column="ThreeID" />
      <one-to-many class="Four" />
    </set>
  </class>

  <class
    name="Four"
    dynamic-update="true"
  >
    <id name="ID">
      <generator class="NHibernate.Id.GuidCombGenerator, NHibernate" />
    </id>
    <many-to-one name="Two" column="TwoID" not-null="true" />
    <many-to-one name="Three" column="ThreeID" not-null="true" />
  </class>
</hibernate-mapping>

using NHibernate;
using NHibernate.Cfg;
using NHibernate.Tool.hbm2ddl;

static class Program {
    static void Main() {
        Configuration config = new Configuration();
        config.Properties.Add("hibernate.connection.provider", "NHibernate.Connection.DriverConnectionProvider");
        config.Properties.Add("hibernate.dialect", "NHibernate.Dialect.MsSql2000Dialect");
        config.Properties.Add("hibernate.connection.driver_class", "NHibernate.Driver.SqlClientDriver");
        config.Properties.Add("hibernate.connection.connection_string",
                              "Data Source=(local);Integrated Security=SSPI;Initial Catalog=test2");

        config.AddAssembly("Test");
        new SchemaExport(config).Create(true, true);

        using (ISession session = config.BuildSessionFactory().OpenSession())
        using (ITransaction transaction = session.BeginTransaction()) {
            for (int i = 0; i < 3; i++) {
                One o = new One();

                for (int j = 0; j < 3; j++) {
                    Two tw = new Two();
                    Three th = new Three();

                    for (int k = 0; k < 3; k++) {
                        Four f = new Four();
                        tw.Fours.Add(f);
                        th.Fours.Add(f);
                        f.Two = tw;
                        f.Three = th;
                    }

                    o.Twos.Add(tw);
                    o.Threes.Add(th);
                    tw.One = o;
                    th.One = o;
                }
               
                session.Save(o);
            }

            transaction.Commit();
        }
    }
}