Can a Warlock cast a spell using a lower level slot than the slots they have available?
For example, if a 9th level Warlock (who has 5th slots) comes into a possession of a Shield Guardian (who can store a spell of 4th level or lower), can the Warlock cast a spell using a lower slot for storage into the guardian?
From the warlock's Pact Magic feature, from PHB p. 107 or here in the basic rules (emphasis mine):
The Warlock table shows how many spell slots you have. The table also shows what level of those slots is; all of your spell slots are the same level.
In the section on casting a spell at a higher level, from PHB p. 201 or here in the basic rules (emphasis by @Carcer):
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting.
