What Hit Dice do multiclass characters spend at the end of a short rest?

Question

Under the rules for Short Rest you are able to heal:

A character can spend one or more Hit Dice at the end of a short rest, up to the characters maximum number of Hit Dice, which is equal to the character's level. For each Hit Die spent in this way, the player rolls the die and adds the characters Constitution modifier to it. The character regains hit points equal to the total.

I have a multiclass character - level 1 Monk (d8 Hit Die), levels 2 - 4 Wizard (d6 Hit Die). What Hit Dice would I roll for healing?

1. Up to 4d8
2. Up to 4d6
3. Up to 1d8 and up to 3d6
4. Any combination of d8's and d6's up to a maximum of 4 dice

Answer 1

Number 3. From page 163 of the PHB:

You add together the Hit Dice granted by all your classes to form your pool of Hit Dice.

[...]

If your classes give you Hit Dice of different types, keep track of them separately. If you are a paladin 5/cleric 5, for example, you have five d10 Hit Dice and five d8 Hit Dice.

Answer 2

You get one hit dice per level of each class that you have. So if you are Wizard 3, Monk 1. you would get 1d8, 3d6 to spend how you want. Hassassin's answer has the pertinent rules quote.

So while the number of Hit Dice you have is equal to your overall character level (4 in this case). The denominations of those HD are dependent on the classes you've taken. This has the function of modeling the increased fragility of taking wizard levels and durability of taking fighter levels (etc).

Also, when you level, you add the HD for the new level (whatever class it is) to your pool of HP. So if you are a Fighter 4 and take a level of Wizard, your HD that you roll at level up is a d6.