what is the percentage of trees currently growing in comparison to 4000 years ago on the planets surface?

Answer 1

The world as of the year 2000 had 3 869 455 000 (3.8x10^9) hectares (ha) forest out of 11.2 billion (10^9) hectares of biologically productive land or nearly 30% coverage currently. Since 1990 to 2000 this is an estimated annual loss of 0.22%
About half of the world's forest area is in the Russian Federation, Brazil, Canada, USA and China, combined. Russia, with nearly twice the combined forest coverage of Canada and US, had an increase in forest cover as has the US.

http://www.fao.org/docrep/004/y1997e/y1997e1r.htm

For prehistoric values the book 'Deforesting the Earth: From Prehistory to Global Crisis' by Michael Williams is a good information source.

Recent Historical Scale of Forest Cover
The Worldwide Fund for Nature published a report stating "we have destroyed 10% of the world’s Forests.”
"Forests with closed canopies now cover about 29 million square kilometres, four-fifths of their extent at the beginning of the 18thC."
http://www.geocities.com/carbonomics/MCsppub/11sp06/11sp06b.html
“One thousand years ago, 80% of central Europe was forested.”
"The sahara was once a luxurious Forest teeming with a wide array of Wildlife .. “the original forests that once stretched from Morocco to Afghanistan even as late as 2000 BC ...”