Of these four nouns the one that confuses English learners most is definitely travel.
Travel is used to refer to the activity of travelling in general. In this meaning it’s always uncountable, i.e. takes no article and is always singular.
His job involves a great deal of travel.
Her interests are history, art and travel.
Travel can be plural when it means journeys, especially abroad.
On their travels they met lots of interesting people, many of whom became their lifelong friends.