Accuracy of Garmin Instinct against Apple watch 4

Do you mean walking outdoors with GPS activated (Walking or Running activity), or just plain walking without any activity started? In the first case (GPS) you can verify on the map which of your devices is more accurate. In the second case (steps only) or in a mixed case (steps + GPS), you may need to measure and correctly set in your device settings the length of your step.

If data during an activity using GPS alone is inacurrate, you may try activating GPS+Glonass or GPS+Galileo in the settings of given activity on your watch (separate settings for Walking, Running, Outdoor Swimming, Bycicle, etc.). You may also consider turning on or off the 3D distance - that can make a difference too, especially if you move a lot in hilly terrain.

I wanna add that the device learns your stride length while using GPS while walking, making it more accurate in difficult situations (high buildings, dense tree cover) or without GPS.

Many reviewers found the Instinct undershoot distance.

Just for info. 
I go for a speed walk of 28 k. To be done in 4 hours. 
Both watches on GPS in outdoor run. 
Both watches on auto stop. 
Right arm Apple Watch 4. 
Left arm Garmin Instinct. 
Walking main streets traject. No turning and twisting forest walks. 
After just under 4 hours my Garmin shows 28k. My Apple Watch shows 29k. On the map both tracks show quite accurate. Correct side of the streets, no corner cuts. 
My Apple has been tested. Is very accurate. (I’m not a fan of Apple Watch. The glass will break within a year. Apple expects customers to buy a new Watch every year).
For the Garmin Instinct, quite ok, only thing, distances are always 3% less then actual distance.
Tested under various conditions. Also forests tracks. Same less distances as compared with Apple Watch. 
 

I and my friend run same speed and with same distance together several times. Because in Apple Watch, there is no built-in function for running and he has to use Strava on his AW4. And recorded distance of the AW4 always longer than of my Instinct. Viewing the gps tracklog in Strava map, mine is smoother than in his Strava. I dont think AW4 is more accurate than Instinct. One built as smartwatch vs one built for sport. 

Same thing happen to me, could you check if sudden pace drops are happening to you too as they are happening to me?

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/instinct/152758/gps-accuracy-issues

The drops in pace could be easily caused by the loss of GPS signal - for example in narrow streets, near walls or hirise buildings, in underpasses, or under trees. Try activating GPS+GLONASS or GPS+Galileo, there is a slightly higher chance to get a signal from other satellites that remain visible. That told, it will drain your battery a little bit faster.

Hi, sorry for the pic quality, as you said any obstructions to GPS "view" of the watch might affect the signal, however All this runs has been made with another garmin in the other wrist (FR 25) and there's no interruptions in the pace chart.

As for the settings currently using in GPS+GLONASS after trying both GPS alone and GPS+Galileo (same results). Also recording is set in "every second" mode.

What I'd like to know if this is a common issue in other Instincts or if it's my unit that might have some problems or just being defective.