On-line trial testing for fourth-grade students was an exploratory study realized as a part of the project "Developing annual test of students' achievement in Nature & Society" realized by Institute for Education Quality and Evaluation. Main ideas of the study were to explore possibilities for on-line testing at national level in Serbia, and to explore the trial test characteristics on a convenience sample in partially controlled testing conditions. This report presents evidences concerning possibilities for on-line testing as well as detailed analysis and comparison of results obtained from the annual and the trial test. In total, 926 students from 50 elementary schools participated in the trial test. Results of the trial and the annual tests were analyzed using Classical Test Theory and Item Response Theory. We have separately analyzed all alternatives and options in multiple choice and multiple response questions (including non-answers). We have also analyzed differential item functioning and item response time in order to collect additional information about questions and the test as a whole. Appended are: (1) Metric Characteristics of Items; (2) List of Elementary Schools that Participated in Online Testing; (3) Trial Test Sample Characteristics; and (4) Questionnaire for Students. (Contains 40 figures and 72 tables.)[This report is a translated and adapted version of the original report in Serbian available at http://ceo.edu.rs. It contains 40 figures, 72 tables, and 9 references.]