The diffraction limited synchrotron light source envisioned for the PETRA IV project will require strong focusing to produce the small emittances in both planes. The large natural chromaticity together with small dispersion will require very strong sextupoles. In order to cope with high gradient magnets the radius of vacuum chamber tends to be in the range of 10 mm, which is very small compared to the current 40-mm wide elliptic chamber. The impedance element in the PETRA III was scaled down to

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... fit into the smaller aperture so that the short range wakepotential can be computed numerically. For instance the beam position monitor (BPM) was reduced to 60% in dimension so that it can be used in PETRA IV. Even if the actual design of hardware does not exist yet, we assume that generic feature of PETRA III model is still valid. In this paper we report the up-to-date information on impedance model of PETRA IV together with the preliminary impedance budget based on the analytical formula. We also report the specific studies carried out to understand the kick-factor scaling with the chamber aperture whose radius is in the range of 8-12 mm.