The question requires selecting the correct statements regarding the urban environment from given options. The correct statements are:

1. Urban heat island can exacerbate urban flooding by intensifying rainfall intensity.
2. In general, land surfaces in urban areas emit more long wave radiation compared to those in rural areas, and thus contribute to higher night time temperature.

Let's examine the listed options and provide explanations:

1. Urban heat island can exacerbate urban flooding by intensifying rainfall intensity.
   • The urban heat island (UHI) effect refers to urban or metropolitan areas being significantly warmer than their rural surroundings. This can lead to increased convection and, consequently, intensified rainfall events, contributing to urban flooding.
2. Urban canyons increase ventilation by trapping heat and thus enhancing urban heat island effect.
   • Urban canyons actually refer to the space between two tall buildings in urban areas. They tend to trap heat, creating a localized heating effect known as the urban heat island effect. However, they do not increase ventilation; rather, they may restrict airflow.
3. Program evaluation and review technique (PERT) is always used to estimate the economic impact of mitigation strategies for urban heat island effect.
   • PERT is a project management tool used for planning and controlling the timing of various tasks in a project. It is not typically employed for economic impact assessments, especially regarding urban heat island mitigation strategies.
4. In general, land surfaces in urban areas emit more long wave radiation compared to those in rural areas, and thus contribute to higher night time temperature.
   • Urban areas often have more impervious surfaces like asphalt and concrete, which absorb and re-emit more long wave radiation than natural land cover found in rural areas. This leads to higher nighttime temperatures in urban environments.

Based on these explanations, options 1 and 4 are indeed the correct statements. Options 2 and 3 contain incorrect or inaccurate assertions about urban environments.