But what if my broker uses just MT?
I'm not criticising anyone whose broker offers only MT4.
I was just trying to suggest relatively tactfully to Esha.J, who is characteristically full of his own opinions but unwilling to listen to anyone else's, that if MT4 is the only one he's ever used, then he has no business offering anyone advice on selecting a trading platform (and incidentally no business at all arguing over it with people who have far more varied experience than he has himself), because by his own admission he has no basis at all for comparison!
Can I trade with that broker with the other platform or is it forbidden?
Sorry, but this is really a question for them. (But be aware that in the murky world of retail forex, some counterparty market-makers who are pretending to be brokers offer only MT4, because MT4-users tend collectively to be the type of client their own business model revolves around attracting: the lowest-hanging fruit in the market makes for a profitable and relatively trouble-free business, from their own perspective).