Let me elaborate a bit more:
I grew up speaking Chinese, I've always hated typing and searching long names. I could never remember if it were "ens" or "ans". Now imagine people who don't use alphabets as their native language, how do you build a brand that is easy for people to find on the Internet? You build a brand with short, memorable, easy to type names or numbers.
3 letters were often used as company acronyms, and the values are easy to understand. 4 letters were plentiful so more investors were attracted to them. Several years ago some Chinese investors figured out that they could map 4 letter phonically to Chinese company names, even common phrases. Some even decided to buy an entire domain name portfolio at an average price of $1500 per name. With an influx of international investors, that $10 LLLL.com domain name 10 years ago is now $350 on the open market. $1000 if they are "Chinese premiums". You can find plenty of articles explains the different "Premium" letters.
As for numbers, again, because they are universal regardless of your native alphabets. The thinking was that because more people could use numbers for their brands, the more valuable they are to the business owners. Just like the 4 letter domain names, some Chinese investors figured out they could map numbers phonically to brands, company names, superstitions, and common phrases.
My strategy is the same over the years. To me short names will always have more value regardless of the craze of the day. I got into 4 letter domain names years ago because I hated typing long words. It just happend that some of my names were "Chinese Premiums".
I sold most of them during the craze. I had a few LLLL . com listed on various markets for BIN price for years, and one day I got notification that one was sold. Before the sale, I considered that particular name to be one of the lowest quality name, so when it was sold so quickly, it caught me by surprise. I quickly pulled all my names from the markets and did some research, and realized what's going on. Over the period about 6 month, I got daily offers for my LLLL names, many tried to lowball me but a few reasonable offers. I sold most of my LLLL names with inbound inquiries. A couple here at NP.
Hope this helps!