4. 为了进行真实细致的考察，他很少乘车坐船，几乎全靠双脚翻山越岭，长途跋涉；

为了：in order to, for the sake of, for the purpose of, with the aim of, with a view to
细致的：careful, delicate, detailed
乘车坐船：“乘坐”应使用一个词 by carriage and boat; take the wagon and the boat
翻山越岭：翻越可使用一个词 climb/cross mountains and ridges
长途跋涉：long difficult journeys

【译文】 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. 为了弄清大自然的真相，他总是挑选道路艰险的山区，人迹稀少的森林进行考察，发现了许多奇山秀景；

弄清：find out; make clear
真相：real situation（区别于表面或造假的情况）；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
艰险：hardships and dangers；dangerous; treacherous; difficult 　　
人迹罕至：untraversed；unfrequented；seldom visited
奇山秀景：grotesque mountains and spectacular sceneries;
奇：grotesque, peculiar, strange, unique
秀：fantastic, miraculous, magnificent, spectacular

【译文】 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. 他常常选择不同的时间和季节，多次重游各地名山，反复观察变换的奇景。

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