Abstract: The purpose of this study is to explore the use of language of learning and teaching on mathematics in grade four. The perpetual poor learner performance in mathematics is a compelling factor in this study. The researcher as the long serving grade 4 mathematics teacher chose grade 4 learners because the transition from grade 3 has impacted negatively in the acquisition of mathematics proficiency. They have just graduated from foundation phase where the language of teaching and learning was the mother tongue. In general, the grade 4 performance in moths showed a significant decline compared to their performance in the foundation phase. Hence there was a need to investigate the impact of language of teaching and learning. Research questions that guide this study aim are to investigate grade 4 learners’ language implications in mathematics, to explore mathematics language that causes poor learner performance, investigate grade 4 experiences in transition from learning mathematics in Setswana to English, and explore the challenges grade 4 teachers are experiencing. The study followed a qualitative research approach with a case study research design. The population in this study consisted of grade 4 learners and three grade 4 mathematics teachers. Random sampling was used to select ten learners from each school and three mathematics teachers. Data was collected through classroom observations, tests, document analysis, and interviews with mathematics teachers. The researcher used a multiplicity of data collection to interrogate the mathematics language proficiency of grade 4 learners. The researcher believes that mathematics is a language on its own therefore using various data collection methods would answer the research questions. During information collection, the researcher discovered that learners who use English from grade 1 to 3 at their schools have the upper hand over learners who use home language from grade 1 to 3. Preliminary findings revealed that learners struggle to understand mathematics concepts when English is used as the language of instruction. Furthermore, learners who used English as their medium of instruction in the foundation phase seemed to have an upper hand over learners who used their mother tongue as their language of teaching and learning in the foundation phase.