As you quote in the question, this text appears under the "Improvised Weapons" heading:

If a character uses a ranged weapon to make a melee attack, [...] it also deals 1d4 damage.

The fact that this appears under the "Improvised Weapons" heading should be a clear sign that such a use of a ranged weapon (whether it originally had the "ammunition" property or not) causes it to be treated as an improvised weapon for that attack.

This section is also referenced and this rule reiterated in the description of the "ammunition" weapon property:

If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon (see "Improvised Weapons" later in the section). A sling must be loaded to deal any damage when used in this way.

Since it is being treated as an improvised weapon, the properties of the original weapon (as well as its weapon type) seem quite irrelevant. The fact that your bow normally requires ammunition and has a certain range you can effectively shoot targets in doesn't matter if you're trying to bash an enemy with the bow or throw it at them. Thus, it seems to be a natural conclusion to me that the weapon is not treated as having those properties for the purpose of that attack.

Rules designer Jeremy Crawford unofficially confirms this interpretation in a November 2017 tweet:

Can you use a longbow in melee to get GWM and Sharpshooter in a single attack for +20 damage?

 

If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.

When used as an improvised weapon (i.e. during the actual attack, not during some other time on the turn), it functions as an improvised weapon. None of its regular weapon properties apply unless otherwise stated.

As you quote in the question, this text appears under the "Improvised Weapons" heading:

If a character uses a ranged weapon to make a melee attack, [...] it also deals 1d4 damage.

The fact that this appears under the "Improvised Weapons" heading should be a clear sign that such a use of a ranged weapon (whether it originally had the "ammunition" property or not) causes it to be treated as an improvised weapon for that attack.

This section is also referenced and this rule reiterated in the description of the "ammunition" weapon property:

If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon (see "Improvised Weapons" later in the section). A sling must be loaded to deal any damage when used in this way.

Since it is being treated as an improvised weapon, the properties of the original weapon (as well as its weapon type) seem quite irrelevant. The fact that your bow normally requires ammunition and has a certain range you can effectively shoot targets in doesn't matter if you're trying to bash an enemy with the bow or throw it at them. Thus, it seems to be a natural conclusion to me that the weapon is not treated as having those properties for the purpose of that attack.

Rules designer Jeremy Crawford unofficially confirms this interpretation in a November 2017 tweet:

Can you use a longbow in melee to get GWM and Sharpshooter in a single attack for +20 damage?

If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.

When used as an improvised weapon (i.e. during the actual attack, not during some other time on the turn), it functions as an improvised weapon. None of its regular weapon properties apply unless otherwise stated.

According to this Sage Advice:

Can you use a longbow in melee to get GWM and Sharpshooter in a single attack for +20 damage?

If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.

When used as an improvised weapon (i.e. during the actual attack, not during some other time on the turn), it functions as an improvised weapon. None of its regular weapon properties apply unless otherwise stated.

As you quote in the question, this text appears under the "Improvised Weapons" heading:

If a character uses a ranged weapon to make a melee attack, [...] it also deals 1d4 damage.

The fact that this appears under the "Improvised Weapons" heading should be a clear sign that such a use of a ranged weapon (whether it originally had the "ammunition" property or not) causes it to be treated as an improvised weapon for that attack.

This section is also referenced and this rule reiterated in the description of the "ammunition" weapon property:

If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon (see "Improvised Weapons" later in the section). A sling must be loaded to deal any damage when used in this way.

Since it is being treated as an improvised weapon, the properties of the original weapon (as well as its weapon type) seem quite irrelevant. The fact that your bow normally requires ammunition and has a certain range you can effectively shoot targets in doesn't matter if you're trying to bash an enemy with the bow or throw it at them. Thus, it seems to be a natural conclusion to me that the weapon is not treated as having those properties for the purpose of that attack.

Rules designer Jeremy Crawford unofficially confirms this interpretation in a November 2017 tweet:

Can you use a longbow in melee to get GWM and Sharpshooter in a single attack for +20 damage?

If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.

When used as an improvised weapon (i.e. during the actual attack, not during some other time on the turn), it functions as an improvised weapon. None of its regular weapon properties apply unless otherwise stated.