(Q4) List the names of the second-level managers of all employees whose rating is "Poor".

/emp[rating = "Poor"]/@mgr->emp/@mgr->emp/name

XPath equivalent to (Q4)

id(id(/emp[rating = "Poor"]/@mgr)[self::emp]/@mgr)[self::emp]/name

Or, for better readability, use this:
id($poorEmpManagers/@mgr)[self::emp]/name

...in conjunction with a variable binding, shown here in XSLT:
<xsl:variable name="poorEmpManagers" select="id(/emp[rating = 'Poor']/@mgr)[self::emp]"/>

• In practice, using ->* would be pretty common, since a valid document may only have one element with a given ID, regardless of that element's name, so I'm not sure how much value the implicit name test really adds beyond the id() function's current capabilities.