I have a key (Long) with autonumbering in a table
but there is a combination of properties that is unique
(String username + Long referenceToOtherTable + Date start + Date end)

how can I do this withoud having a Component like this

<id name="id" .../>
<component unique="true" ...>
<property name="username " .../>
<property name="referenceToOtherTable " .../>
<property name="start " .../>
<property name="end" .../>
</component>

because then I need another class for this component that has no specific meaning, or is this the only way (component + extra class)?

Just map them as properties of your class, there is no need to declare component unless you want to.

I need to have that combination of properties unique
if I put unique in each of the properties; each of them are in their own right unique and that's not what I want

Just define that unique constraint at DB level, and you could check it at class level as well if you want.
Generate DB schema yourself as you need it, do not trust Hibernate to do it.

ok, defining the constraint at database level is not my job but what do you mean by checking it at class level then

I mean validation on save or update.