This study investigated the use of formulaic language, in the form of four-word lexical bundles, by Thai learners of English as a second language (ESL) at various levels of English proficiency during interlanguage communication. The investigation focused on two aspects: the frequencies and pragmatic functions of the four-word lexical bundles. A total of 120 Thai ESL learners participated in the study, ranging from basic to intermediate and advanced English proficiency levels. In terms of frequency, a list of the most frequently used four-word lexical bundles by Thai ESL learners at each level was examined. Results showed that learners at higher English levels used less formulaic language in interactions than those at lower levels. Two similar four-word lexical bundles, centered on "I don't know" and "I think," were used by Thai ESL learners at all three English levels. The functional analysis demonstrated that Thai ESL learners at all three English levels used formulaic language to assert group identity as a device for social interaction and as a device for memory limitations to buy more time and process shortcuts. The function of asserting a separate identity was not used by intermediate-level Thai ESL learners. Furthermore, the function of processing shortcuts was used less at the advanced level than at the basic level. The findings of this study indicate that Thai ESL learners use formulaic language differently at different English proficiency levels. Therefore, formulaic language used by ESL learners in spoken English discourse should be further investigated.