MT4 or MT5 platform
If you don't have any experience with trading, or if you are only doing manual trading, start with MT5.
Use MT4 only if you already have experience and plan to auto-trade with EAs that you already have and depend on.
So, in essence, except for very special cases, best to go with MetaTrader 5!
It uses very little resources from your computer and it works well, even with low internet bandwidth.
I will not dispute the advantages of resource and bandwidth usage, but how can you "feel" that an older technology is more advanced than a newer technology?
As for multiple trades simultaneously, they both support it and response time for order placement is faster on MT5.