Hi Rob, 

I'd try something like this (I'm assuming you want to show all nodes at levels 2-4, not just relative to the current node) - 

@using umbraco.MacroEngines;
@inherits umbraco.MacroEngines.DynamicNodeContext

@{ 
    const int startLevel = 2;
    const int finishLevel = 4; 
    var parent = CurrentModel.AncestorOrSelf(startLevel); 

    if (parent != null)
    {
        @traverse(parent,startLevel,finishLevel) ;
    }
}

@helper traverse(dynamic parent,int startLevel,int finishLevel)
{ 

    <ul> 
    @foreach (var node in parent.Children.Where("Visible")) 
    { 
        <li>
            <a href="@node.Url">@node.Name @node.Level</a> 
            @if ((parent.Descendants(5).Count() != 0 && node.IsDescendantOrSelf(Model) && node.Level <= 5) || node.Level <= 3)
            {
                @traverse(node,startLevel,finishLevel);
            }
        </li>

    } 
    </ul>
}

Seems to work for me - if I'm on a branch of the tree that is four or less levels deep, the navigation lists all nodes at levels 2-4. If I'm on a branch with more than four levels, it lists levels 5 and 6, but only for that branch. I'm checking that a node has descendants at level 5, the node is a descendant of the current node, and then traversing children up to level 5 (to display level 6 nodes). If there are no descendants at level five, then we traverse as far as level 3 (to get the level four nodes)

Should at least be a step in the right direction. Hope that helps.

cheers
Nathan