Armin, thanks for the support.
I used example Receive dump. I have the Uno R4, Tx on the pin 7, Rx on the pin 8. Then I send IrSender.sendNEC(0x4, 0x3, 0); and wait 15s before the same data send again. Here are two dumps. I decided for two as there is always something else received.
#1
17:39:27.368 -> -3276750
17:39:27.368 -> +9700,- 400
17:39:27.368 -> + 550,- 450 + 500,- 500 + 600,- 400 + 500,- 500
17:39:27.410 -> + 500,- 500 + 550,- 450 + 550,- 450 + 500,- 500
17:39:27.410 -> + 550,- 450 + 500,- 500 + 600,- 400 + 500,- 500
17:39:27.410 -> + 500,- 500 + 500,- 500 + 500,- 500 + 500,- 500
17:39:27.410 -> + 500,- 500 + 500,- 500 + 500,- 500 + 550,- 450
17:39:27.410 -> + 550,- 450 + 550,- 450 + 600,- 400 + 500,- 500
17:39:27.410 -> + 500,- 500 + 600,- 400 + 500,- 500 + 500,- 500
17:39:27.410 -> + 500,- 500 + 500,- 500 + 500,- 500 + 500
17:39:27.410 -> Sum: 41600
17:39:27.410 ->
17:39:27.410 -> Result as internal 8bit ticks (50 us) array - compensated with MARK_EXCESS_MICROS=20
17:39:27.441 -> uint8_t rawTicks[65] = {194,8, 11,9, 10,10, 12,8, 10,10, 10,10, 11,9, 11,9, 10,10, 11,9, 10,10, 12,8, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 11,9, 11,9, 11,9, 12,8, 10,10, 10,10, 12,8, 10,10, 10,10, 10,10, 10,10, 10,10, 10}; // Protocol=UNKNOWN Hash=0x95DACC9 33 bits (incl. gap and start) received
17:39:27.441 ->
17:39:27.441 -> Result as microseconds array - compensated with MARK_EXCESS_MICROS=20
17:39:27.483 -> uint16_t rawData[65] = {9680,420, 530,470, 480,520, 580,420, 480,520, 480,520, 530,470, 530,470, 480,520, 530,470, 480,520, 580,420, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 530,470, 530,470, 530,470, 580,420, 480,520, 480,520, 580,420, 480,520, 480,520, 480,520, 480,520, 480,520, 480}; // Protocol=UNKNOWN Hash=0x95DACC9 33 bits (incl. gap and start) received
17:39:27.483 ->
17:39:27.483 -> Pronto Hex as string
17:39:27.483 -> char prontoData[] = "0000 006D 0021 0000 0176 000F 0016 0011 0014 0012 0018 000F 0014 0012 0014 0012 0016 0011 0016 0011 0014 0012 0016 0011 0014 0012 0018 000F 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0016 0011 0016 0011 0016 0011 0018 000F 0014 0012 0014 0012 0018 000F 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 06C3 ";

#2
17:40:03.145 -> -3276750
17:40:03.145 -> +9000,-4450
17:40:03.145 -> +1300,- 550 + 500,-1850 + 500,- 600 + 300,- 700
17:40:03.180 -> + 350,- 650 + 600,- 550 + 550,- 650 + 550,-1750
17:40:03.180 -> + 550,-1700 + 550,- 600 + 600,- 800 + 450,- 500
17:40:03.180 -> + 500,- 500 + 500,- 500 + 500,- 500 + 500,- 500
17:40:03.180 -> + 500,- 500 + 500,- 500 + 500,- 500 + 500,- 500
17:40:03.180 -> + 500,- 500 + 500,- 500 + 500,- 500 + 500,- 500
17:40:03.180 -> + 500,- 500 + 500,- 500 + 500,- 500 + 500,- 500
17:40:03.180 -> + 600,- 400 + 500,- 500 + 550,- 450 + 600
17:40:03.212 -> Sum: 50750
17:40:03.212 ->
17:40:03.212 -> Result as internal 8bit ticks (50 us) array - compensated with MARK_EXCESS_MICROS=20
17:40:03.212 -> uint8_t rawTicks[65] = {180,89, 26,11, 10,37, 10,12, 6,14, 7,13, 12,11, 11,13, 11,35, 11,34, 11,12, 12,16, 9,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 10,10, 12,8, 10,10, 11,9, 12}; // Protocol=UNKNOWN Hash=0x48F5BD2 33 bits (incl. gap and start) received
17:40:03.249 ->
17:40:03.249 -> Result as microseconds array - compensated with MARK_EXCESS_MICROS=20
17:40:03.249 -> uint16_t rawData[65] = {8980,4470, 1280,570, 480,1870, 480,620, 280,720, 330,670, 580,570, 530,670, 530,1770, 530,1720, 530,620, 580,820, 430,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 480,520, 580,420, 480,520, 530,470, 580}; // Protocol=UNKNOWN Hash=0x48F5BD2 33 bits (incl. gap and start) received
17:40:03.285 ->
17:40:03.285 -> Pronto Hex as string
17:40:03.285 -> char prontoData[] = "0000 006D 0021 0000 015B 00AA 0033 0014 0014 0046 0014 0016 000C 001A 000E 0018 0018 0014 0016 0018 0016 0043 0016 0041 0016 0016 0018 001E 0012 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0014 0012 0018 000F 0014 0012 0016 0011 0018 06C3 ";

Thank you.

Sorry I have no clue.
There is obviously one (the first) bit missing, but this should not be R4 specific (or the WiFi part?).
Without a R4 at hand I am stuck.

@ArminJo , thank you for your help anyway. I actually work as an embedded SW guy, so I know how headache it is try to analyze something without being able to debug it on the target HW.
I will try to look myself. I have no previous experience with the IRRemote library and also Arduino in general, but at least now I have a reason to dive a little bit into it.
I will post here if I have some relevant findings regarding this topic.

@DavidSimacek did you manage to solve your issue? Mind sharing here how you solved it?
Thanks