I had an old tape recorder from Sanyo.I took out the speaker and want to use it to play sound through Arduino UNO.The speaker is rated at 4ohm 25W. So can I use it with some kind of amplifier? Give me a guide or any link .Thanks 
So can I use it with some kind of amplifier?
Yes it has to be used with an amplifier.
AdaFruit have suitable ones.
I have one of [u]these[/u]. (Note that you have to buy the power supply separately for this particular amplifier.)
to play sound through Arduino UNO.
The easiest thing is to use regular [u]powered computer speakers[/u], which have a built-in amplifier.
rated at 4ohm 25W.
25 Watts is kind-of "big" for a tape recorder speaker... Are you sure it's not 0.25W? or 2.5W? (It's not a problem, the power rating is the maximum and you don't have to feed it 25W.)
How about a LM386? it's easily available here.