spells beyond a casters ability.

[Darth Stabber]

For the sake of argument, let's say we have an assaassin with and an int of 12. And let's assume that he has no interest +int gear.

For the third and fourth level spells known could he instead pick extra first and second? For the spell slots could he prep lower level spells into them?

Would this be a fair "on the spot" house rule: additional 3rd level spells known may be spent on first level spells, and those 3rd lvl slots are now 1st level, ditto with 4th to 2nd.

Or should that be the price you pay for being a dumb assassin?

[Nettlekid]

That rule is more or less in place already, but less restricting. Whenever a caster gets spells known or spells per day, they can always use a higher level slot for a lower level spell. If a Sorcerer were to get a new level 4 spell, he could actually learn a level 3 spell instead, and forever have one fewer 4th level spells known and one more 3rd level spell known. And you can use a 9th level spell slot to cast a cantrip, if you so choose, except that it doesn't increase DCs or anything unless you were to Heighten it, at which point it becomes a higher level spell rather than just being used in a higher level spell slot.

[Glimbur]

To back that with a rules quote,
Spell Slots
The various character class tables show how many spells of each level a character can cast per day. These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower-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.

[Chronos]

The rules do allow you to use a high-level slot for a low-level spell, but I'm not aware of anything that says you can do the same for a spell known.

Were I DMing, I would strongly discourage doing so, since if you ever changed your mind, your character is permanently nerfed. But if a player convinced me that they really meant it, I can't see any reason not to allow it.

[Erik Vale]

Also, look into arcane manipulation, that way you get more for using your higher levels as lower levels [search dndtools, I don't have it handy. It's a feat though.].