The purpose of this study is to smooth temperature oscillation of a 4 K Gifford-McMahon (G-M) refrigerator. A cold-head of G-M refrigerators have temperature oscillation because of reciprocating of a displacer. In particular, the phenomenon becomes large bellow 15 K level. The reason is that specific heat of regenerator materials generally decrease with decreasing temperature. The tested two-stage G-M refrigerator has temperature oscillation of 0.33 K (peak to peak value) at 4 K (middle value of temperature oscillation). To smooth temperature oscillation of a cold-stage, we carry out by a simple method: Sandwiching a suitable material between a cold-stage and a cold-head allows smoothing temperature oscillation at the cold stage. In this research, a stainless plate is sandwiched. When using a stainless steel plate of 2 mm thickness, temperature oscillation of the cold-stage is reduced to 0.06 K at 4 K. The result is in good agreement with the calculated value.