Is it permissible to combine the Dhuhr and Asr prayers by performing them earlier (jama' taqdim), and the Maghrib and Isha prayers by performing them later (jama' ta'khir), due to the difficulty of performing ablution and finding a place to pray at work?

We do not know the nature of your work, but performing prayer does not require a great deal of time and effort that would prevent one from performing it on time. It can be performed in any pure place. If you do not find water for istinja’ (cleaning oneself after relieving oneself), istijmar (using stones or other pure objects for cleansing) is sufficient for you. For wudu’ (ablution), you can use hand-washing areas. If you are completely unable to find water, tayammum (dry ablution) is permissible for you. Combining prayers is not permitted except in cases of dire necessity, such as travel or rain; otherwise, it is one of the major sins.