I'm not entirely sure I understand bonus spells...

[...Eh?]

Is there a minmum level you have to be, or can a first level wizard with 18 Int really cast level 4 spells? If so, that makes it possible to qualify for the prestige class Master Tranmogrifist at second level. (Although, since it's centered around Polymorph, which has an HD restriction, it;s kinda useless to take it then.)

[PaladinBoy]

If you can cast spells of the correct level already, or the "Spells per Day" line has 0 for that level, then you can cast bonus spells of that level. Otherwise, you can't cast those bonus spells.

[Corolinth]

http://www.d20srd.org/srd/theBasics....ndSpellcasters

In addition to having a high ability score, a spellcaster must be of high enough class level to be able to cast spells of a given spell level.

Note the bard:
http://www.d20srd.org/srd/classes/bard.htm
At level 2, he has 0 level 1 spell slots. He knows 2 level 1 spells, with the following proviso:

1. Provided the bard has a high enough Charisma score to have a bonus spell of this level.

At the same level, he has <blank> level 2 spell slots. He knows <blank> level 2 spells. Regardless of his charisma, he does not know, and therefore can not cast, any spells of level 2 or higher.