In this situation, can you make a U-turn immediately after this sign?

In the photo you are approaching a bridge with white and blue barriers, and on the right there is sign F-4 with the name of the river "Wisła" (the Vistula). Immediately beyond this sign, that is on the bridge, you cannot make a U-turn. The answer is: no.

The Regulation on road signs and signals states that sign F-4 gives the name of the river or other watercourse over which the road passes, so it informs you that you are entering a bridge. The Road Traffic Act prohibits making a U-turn on a bridge, on a flyover and in a tunnel, and also anywhere it could endanger road safety or obstruct traffic. Sign F-4 itself does not prohibit anything, but the structure it announces is a place covered by the statutory ban. So you plan your U-turn before the bridge, if conditions allow it, or only after it, at a junction.

Legal basis: Art. 22 para. 6 point 1 and 4 of the Road Traffic Act (Prawo o ruchu drogowym); § 69 para. 4 of the Regulation on road signs and signals