MT4 PROS:
1. Yes, it is user-friendly
2. Stable
3. Allows automated trading with a huge community behind it and thousands of free EAs available.
It does lack some things which may or may not be important to you, though.
CONS:
1. Charting is pretty good, but trend lines often look different on different timeframes.
2. Trailing stops are client-side, which means your MT4 terminal must be open for them to work.
3. No If/Then or OCO orders available, making it tougher to apply certain trading styles.
4. Some of the MT4 brokers are scams, and MetaQuotes knows it, yet does nothing about it.