Which tones are hardest, why, and where to start.
Indonesian is non-tonal (intonation only), so all four tones are new categories. Tone 2 is hardest, followed by Tone 3, and T2 and T3 are predominantly misidentified as each other. In connected speech learners rely more on duration than on f0 contour, which shifts the difficulty toward Tone 4.
Source: Chow, Liu & Ning (2018), The Perception of Mandarin Tones by Thai and Indonesian Speakers, TAL 2018, 197–201. DOI: 10.21437/tal.2018-40
Group averages tell you where to look, not what you actually do. Two speakers of the same first language can differ a lot. The only way to know is to record yourself and compare.
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