my setup is like this.
interface 1= Wan
interface 2=LAN-TO -AP 1 (HOTSPOT BROADCAST)
interface 3=LAN-TO -AP 2 (HOTSPOT BROADCAST)
interface 4=NOT IN USE

I want to Bridge Interface 1 and Interface 4 so that i can provide a internet connection to a client without hotspot and i can simply limit the INterface 4 bandwidth to 1MBPS from winbox router interface. but what i want is that the client should be able to browse/downloda anything at 256kbps and only youtube at 1MBPS so that there's no buffering.Plz provide me help with this. THANK YOU.

Did you first read the many other postings about the (im)possibility of treating Youtube different from other sites?
There still are some tricks possible but they are not for the beginner...

Will simple ques work on interface 4?

It is no problem to use queues, of course when you want to separate the interfaces take them out of the switch.

However, it is *very tricky* to classify traffic as "Youtube" or "not Youtube".
Why? Because Youtube has started using encryption. Therefore you will find many simple solutions that tell
you to just use some string match at L7, but these no longer work because the traffic at L7 is no longer in plain text.
So, you have to do nasty things like finding all addresses for youtube and putting them in an address list, or
sniffing the DNS traffic to see when a youtube request is made and try to find the IP that it returns.
The latter is even more difficult for traffic classification than it is for purposes of "blocking Youtube", another
request often seen on the forum.

HMMM....

Thank YOU