Why is pound a stronger currency than US dollar?

Just because a currency is worth more than one US dollar does not mean
that it is stronger than the dollar.  It is simply a matter of how the
nation's central bank defines its currency.  A few countries with
rampant inflation (Turkey, Brazil, France) have reset this when all
prices were in thousands by striking off the three zeros, maybe also
renaming the currency.

Until 1949, the pound was almost always worth more than 4 US dollars.

If pound is not stronger then how is the currency of a country valued
in the international market

The relative value of the pound to the dollar (or to any other
currency)  - the exchange rate -  results from daily (now around the
clock) trading by foreign currency dealers (usually on behalf of
financial institutions) based on the relative demand for the one or
other currency.  This demand can be influenced by economic and
political news.  For example, if interest rates rise in one country,
the exchange rate (price) for its currency may increase as people
choose to buy it and hold funds at the higher interest rate.  Or,
hopefully theoretical, if the US were to bomb Iran, the value of the
dollar might decline.

Does that help?

I think $2.00 is not enough to recieve a befitting answer

Asiyah,
Comments are never an official answer that would cost you the price of
your question (just the $0.50 charge by Google).  Only G-A Researchers
with a blue user name can post "answers", so my attempts to answer
your question have not cost you two dollars.
Regards, Myoarin