I've found that one of the real challenges in learning Mandarin is understanding idioms. If you aren't familiar with the term "idiom", one dictionary defines it as "a group of words established by usage as having a meaning not deducible from those of the individual words". In Mandarin, the word is 成语 (Chéngyǔ) which is literally "become" or "make", then "language". The term also applies to proverbs in some contexts.
A good example of this is the expression:
Literally that means "horse, horse, tiger, tiger". But if you check the Google translation for that, you'll see it means "sloppy". From how I've seen it used, I'd put it closer to "careless". I've had feedback that it's closer to "just so-so".
The basic story that I've heard is about a painter who was painting a tiger and was asked to paint a horse instead. He was too lazy to start another painting, so he just changed it to a horse. When one of his sons asked what the painting represented, he said that it was "a tiger", but he answered "a horse" when another son asked.
Now I've heard variations of that story but it's a good example of an idiom. And note that most of these idioms are four characters long.
The trick is that most of these stories are already known to the majority of the Chinese population. They typically relate to well-known stories or historical quotations.
With 马马虎虎 (Mǎmǎhǔhǔ) , I've asked about how often it's used today. I get different answers. My teacher and others say they use it in daily conversation, but I've heard others say that it's only new learners that ever say it. It's not an age thing. My teacher is in her thirties and has very current language.
Like with any language, even short expressions are often shortened again. You'll find the two character version 马虎 (Mǎhǔ) commonly used for "careless".