In the Players handbook it says this about Spell slots:

A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell. For example, a 7th-level wizard has at least one 4th-level spell slot and two 3rd-level spell slots (see Table 3–18: The Wizard, page 55). However, the character could choose to prepare three 3rd-level spells instead, filling the 4th-level slot with a 3rd- level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his or her due still gets the slots but must fill them with spells of lower level.
This section is mostly about Wizards and preparing spells. Can a spontaneous caster like a sorcerer, also cast a 3rd level spell out of a fourth level slot? (e.g. if I need that 3rd level spell, but have used up my 3rd levels slots for the day, can I cast it out of a 4th level slot)?

Or do you need the Heighten Spell feat to do that? Or Does Heighten Spell just ensure your can use the Higher DC of that higher spell slot?