As the title says: Is a 1st-level spell cast using a 4th-level slot a 1st-level spell, or a 4th-level spell?
The spell is a 4th level spell. (PHB, p.201)
Casting a Spell at a Higher Level
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. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level. Effectively, the spell expands to fill the slot it is put into.