Electronic – Charging laptop using external 18650 cells

Laptop batteries are usually "smart batteries" which means they have in-built protection for the whole pack. That's why they use unprotected batteries.

Protected cells of type 18650 are also not exactly the same dimensions as unprotected batteries because of the extra PCB on one end. These are usually used in flashlights and other gear that use a spring to deal with the difference in size, but since laptop battery cells are in a close space welded together, you might find that protected cells will not fit.

From Wikipedia:

Lithium-ion cells are made in various sizes, often assembled into packs for portable equipment. Many types are also available with an internal protection circuit to prevent over-discharge and short-circuit damage. This can increase their physical length; for example, an 18650 is around 65 mm (2.6 in) long, but may be around 68 mm (2.7 in) long with an internal protection circuit. Safe and economic recharging requires use of chargers specified for these cells. Popular applications include laptop battery packs, telephones, electronic cigarettes, flashlights and cordless power tools.

Cells in devices that you do not wish to be a flaming ruin MUST have protection.
Having protection circuits does not guarantee it will not happen.

Cells without protection are intended for use by either manufacturers or experts or enthusiasts who add their own or for suckers.

Whole device protection and cell protection are complementary and serve overlapping but different roles.

4,000 mAh 18650 LiIon batteries are ~~~= 99.9% +0.1% - 0.0% sure to be rubbish.
ie not just < 4000 mAh but << 4000 mAh and low quality.
The people who bogus label cells

• almost never feel an obligation to use the best cells they can and

• almost always decide to add injury to insult by using junk as well as lying.

Real world experience shows that the value of 'almost' is very high in both cases.

Higher capacity cells can usually be fitted OK.
MUCH higher capacity will lead to long CC tails and overcharge but not an issue here as mAh_new is < to << 4000 mAh.

ADDED:

Notice that in this ad and all their other ads they ALWAYS show non-brand-label views of the battery.

However, you may find that the racing stripes and general colour scheme a good match for the well known "Ultrafire" brand batteries. This may in fact be a real brand and these may be real examples of it BUT you can buy empty shrink wrappers to apply to the battery of your choice with this (or other) branding on it , so caveat emptor. Better nullius emptor I'd hazard.

These ones are a stunning 6800 mAh - a steal at the price. Available here

You'll find others similarly arrayed here and
here - 3000 mAh and 4000 mAh and

6800 mAh !!!!!!!!!!!!!!!! {again}

and unspecified but with GENUINE CREE 2000 lumen {so 20+ Watt} flashlight for $9.27 and
only 4000 mAh and
that's better - 4200 mAh
and .............

Flee!