can any one recommend a method to encode three or four letters?

what I mean 'encode three or four letters' is

ok let me explain the situation.

I have three letters , for example, CAT

but I don't want to write it as CAT on a paper because CAT could mean something important to me, it could be a password, but it's just an example.

at first, I thought about writing CAT as DBU which moves each letter forward by one
i.e. C->D ; A->B ; T->U

but I think it's quite easy to be broken by guess or brutal force. Especially I have quite a number of such words that I want to encode them before writing down on a paper, if one of the them is guessed, then all other words would be easily cracked.

I'd like to know if there is more difficult strategy for encoding words with three or four letters??

Learn a language normal theves don't know. Otherwise use some simple method you think of and just pick a spot to store the them where it isn't obvious to be stored (desk drawer, in a medicine drawer, etc)

Any simple letter swapping cypher is relatively easy to crack now. The simplest answer is don't write down your passwords especially on paper at your desk

You could create your own encoding algorithm that only you will know yourself.

Let's say you map all letters A-Z to the numbers 1..26. You could have a function which takes a letter, divides by some number, and do something with the remainder (modulus). This isn't a linear cypher as mentioned in your post, and harder to crack (you'll need to remember the divisor).

Or, you could hard-code the letters A-Z to any random numbers (like A is 12, B is 30) and work with that. In either example, a stranger wouldn't be able to crack it too easily. It really depends on the technical knowledge of who you're trying to hide it from. An experienced software engineer could take minutes, a layman may never figure it out!

"It could be a password?" So are you wanting to send encrypted messages to a prisoner? LOL. The FBI has a very good cryptanalysis lab and can break a lot of hand written cyphers.

I would use an AES encryption program and store the files. But this isn't hand writing because you will have to use a computer to decrypt it. Here's one program. http://sourceforge.net/projects/simpletextenc/

There are many others and online encrypts as well. Just Google AES text encryption. You could even store text and files in files. This is called Steganography. Some Steganography programs don't really hide the text though. http://sourceforge.net/directory/os:linux/freshness:recently-updated/?q=Steganography

I have used Openstego.

By far the best EVER written code is the one time pad. http://users.telenet.be/d.rijmenants/en/otp.htm

https://www.fourmilab.ch/onetime/otpjs.html|||||||||||| http://krako.chez.com/nouveau/spy/cs013.htm

Onetime pads are still used to this day by spies in the field and if you have a shortwave radio you can hear what are called numbers stations that sound like this giving out coded signals to spies. https://www.youtube.com/watch?v=uFT5y17f1sU

Now why would a Russian station give coded messages in Spanish? Hmmmm...

Here are some Onetime pad programs. http://www.hacker10.com/encryption-software-2/list-of-one-time-pad-encryption-programs/

One time pad net looks pretty nifty. If you use it be sure to save your pad and then load the pad to decrypt. I would use a pad size of at least 50.

We'd probably be able to give you a better answer if we knew what it was you were trying to encode and when you'd need to be able to decipher what you're using.