4. 為了進行真實細致的考察，他很少乘車坐船，幾乎全靠雙腳翻山越嶺，長途跋涉；

為了：in order to, for the sake of, for the purpose of, with the aim of, with a view to
細致的：careful, delicate, detailed
乘車坐船：“乘坐”應使用一個詞 by carriage and boat; take the wagon and the boat
翻山越嶺：翻越可使用一個詞 climb/cross mountains and ridges
長途跋涉：long difficult journeys

【譯文】 He seldom traveled by horse cart or boat but climbed mountains and ridges and took long, difficult journeys most of the time with the aim of conducting a real and careful exploration.

5. 為了弄清大自然的真相，他總是挑選道路艱險的山區，人跡稀少的森林進行考察，發現了許多奇山秀景；

弄清：find out; make clear
真相：real situation（區別于表面或造假的情況）；the actual state of affairs; fact; truth
真相大白come out in the wash; The truth is out; The facts are clear now.
弄清真相：get a clear picture of
艱險：hardships and dangers；dangerous; treacherous; difficult 　　
人跡罕至：untraversed；unfrequented；seldom visited
奇山秀景：grotesque mountains and spectacular sceneries;
奇：grotesque, peculiar, strange, unique
秀：fantastic, miraculous, magnificent, spectacular

【譯文】 To get a true picture of nature, he picked/chose mountains with treacherous pathsand unfrequented jungles for explorations, discovering many grotesque mountains and beautiful sceneries.

6. 他常常選擇不同的時間和季節，多次重游各地名山，反復觀察變換的奇景。

【譯文】He would revisit famous mountains at different time and in different seasons to observe the changing magnificent views time and again.