A fourth level crossblooded sorcerer can cast at least three spells of second level, yet due to being crossblooded, knows one less spell of each level than a standard sorcerer. In particular, at fourth level, the sorcerer knows precisely zero 2nd level spells.
Does the sorcerer still have access to their second level spell slots?
(If yes, then there is another question which concerns using those slots: Lower spells in higher spell slots )