Hi mate,
I currently use 4 and have done for a long time now. Looked at 5 in some depth but have no reason to change as my personal indicators and management ea are programmed in mql4. They do not share the same language so you cannot interchange code.
If you dont use custom written indys/eas then you can take your pick. 5 does have more features, you are not limited to the standard 9 timeframes, there are more standard indicators, there are now 5 order types etc etc etc
I would go for 5 unless you have specific attachment to 4 which it sounds like you do not.
Maybe try both as you can download both from your broker surely?