• as time flies, windows or Mac os get updates, and new computers comes with new OS, old wacom never expect that in future when it was released
• so Don't Expect Latest Wacom Driver to work with your OLD wacom,
• go to https://www.wacom.com/en-us/support/product-support/drivers
• and search the name of your wacom for last latest avaible driver for it.
• Actually, a wacom driver can support certain range of multiple generation of wacom,
  if you happened to have 2 or more generation of wacom, you may need to see the release note > expand “Compatible products” to see supported device for that driver
  • for example,
    • last driver for PTK-840 is: Driver 6.3.41-1,
      • which support PTK-440, 640, 840, 1240, PTH and PTK-450, 650, 850, PTH-451, 651, 851
    • last driver for PTH-651 is: Driver 6.4.0-11,
      • which support PTK-1240, PTH-451, 651, 851
    • as you can see, if you use Driver 6.4.0-11, then your gen 4 wacom can't be used
    • thus you need to install Driver 6.3.41-1, which both your PTK-840 and PTH-651 can use together
• Note, you can't have 2 drivers working at same time, you need to uninstall one driver to install another driver
• if you also use old non-pro lines wacoms, like my old bamboo fun medium (last available driver is 5.xx), so there may be big gap in drop support, as pro lines generally got longer support time range to fit multiple gen support.

• it happens sometimes, from ancient wacom time till modern times
• you can go services window and stop and restart wacom service, or just run my batch script for windows, click and save fixWacom.bat, (make sure its name download correct, ending with bat with gear icon) and right click on it to run with admin rights

fixWacom.bat

@echo off
net stop WTabletServicePro
timeout /t 5 /nobreak > NUL
net start WTabletServicePro
timeout /t 2 /nobreak > NUL

ref:

• https://machollywood.com/blogs/news/wacom-tablets-and-cintiqs-with-compatible-pens#tablets
• https://picmod.com/goods.php?id=140
• https://en.wikipedia.org/wiki/Display_size
• https://www.brother.co.uk/support/answers/is-a3-bigger-than-a4

How to Read the Model Name

• [ModelPrefix]-[Size][Generation][SubVersion]-[addition feature/color]

Note and Tips

• you don't need your wacom size to match your screen size
• you don't need your wacom size to match your paper size unless you trace paper
• for 24/27“ dual screen setup: medium size is best, small size will fly mouse too quickly but usable as mouse
• for 24” screen: if you use medium size, you will feel very slow to move like mouse, but ok for drawing details, single small size will feel more balance between as mouse and drawing.
• any screen smaller than 24“, just use small wacom is enough.
• large size is not recommended to use as a mouse, your arm will feel tired to move from screen corner to corner.
• compared with wacom screen version, tablet version works more like mouse and pen, while screen mainly use like pen input, with your hands on screen all the time.
• compared to the lower line models, like bamboo and non-pro lines, they are more portable to use with travel laptop or as cheap alternative, but the pen is too slim for long time work use; and buy 2nd hand old gen wacom will be much cheaper and better to buy new cheap versions.
• wacom tablet can use for more than 10+ years, if you take care the draw surface, put cover when not use.
  (my old bamboo fun medium has served me well for at least 16 years, I gave it up later due to its sensitive sensor aged and behaved a bit shaky when in idle. also, I got cheap 2nd hand pro version wacom to replace it.)