Practice the th sound by putting your tongue between your teeth and blowing air over it. Make sure you hold this sound at the end of the word.
You can try other words with this sound, like with, both, earth, teeth.
You might also find that the vowel in fourth is difficult to say correctly. “Or” is really two vowel sounds together: o and er. Try saying it slowly. Sometimes people leave out the “er” sound or make more of an “uh” sound instead. Avoid saying oh-uh by making sure you have a tight tongue pulled back in your mouth to say er.
Now try putting it all together.
Happy Fourth of July!