UK International mail comes under "Letter" or "Small Packet". "Large Letter" is for domestic mail only.
The price as they say is right however {up}

Ooooh thank you SO much for your help!

So I believe it shouldn't weigh anymore than 30 grams (in an 8 inch x 10 inch envelope). So I THINK if I'm understanding correctly, it'll fall under the "large letter" category, and cost around £1.88 to mail back to the US, in which case 4 second class stamps should work.

If anyone could correct me if I'm wrong or clarify if that sounds right, it'd be fantastic. x.x

Hi, Yes 2nd class stamps are fine for airmail letters, as long as you meet or exceed the postage cost, and are currently worth £0.50 each. These are like forever stamps so if the prices go up in April, so does the value of the stamp.
You can find the current prices here http://www.royalmail.com/sites/default/files/RM_Our_Prices_OCt2013.pdf
or use the price calculator http://www.royalmail.com/price-finder
Roughly knowing the weight of the return item and converting to grams will help {up}

Hello! I'm hoping I can get a bit of help.

I live in the USA. Previously, I mailed a letter to an actor in the UK, and on my SASE I put 4 second class stamps (I'm a newb, and that's what a seller told me would work). It was successful, and I received a reply soon after.

I'm looking to send a letter to the UK once more, which should weigh practically the same as the last. However, when I went to put my second class stamps on the envelope, the stampbook said they're no good for international mail. Is this true? If so, I don't understand how I got a reply last time.

Also, if the second class stamps [i]will[/i] work, is it right that the prices have gone up, and that I'll need more than 4 of them now?

I'd really appreciate any help!