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So I have a Carcharadon SM army planned out for a small force.
Now I want my regular tac marines / space marine troops to be armed with bolt pistol + chainsword (going with the fluff).

But it seems like the MK IV and III SM box description does not state they can be armed with Bolt pistols

This multi-part plastic kit contains the components necessary to assemble a ten-man Space Marines Tactical Squad in Mark IV power armour, with an impressive selection of armaments including bolters, missile launcher, heavy bolter, chainswords, power swords, power fists, plasma pistols, lightning claw blades, combi flamer, melta and plasma guns, a melta bomb and a set of combat blades! These miniatures also include ammo packs, holstered bolt pistols and a selection of frag and krak grenades. Supplied with ten Citadel 32mm Round bases and a transfer sheet.

Quoted from the GW online store.

So it seems like they only have holstered bolt pistols...

How would I put the MK IV and III marines to have chainswords and bolt pistols?
(like regular Assault Marines)

Just go online and buy assault weapon bits. The arms themselves may be a little different, but not enough for anyone to care. Alternately I believe Forgeworld makes arms or assault sets for them. Personally I just mix whatever I want.

The sprues for each come with a couple of officer/sergeant weapons (I think a single bolt pistol arm, and a single plasma pistol arm, and a chain sword and maybe power sword) but not enough for the squad. Sadly the MkIII stiffs you with the chainswords as they're molded to be hip-mounted and you'd have to green stuff them to make them work as normal chain swords.

You could buy the Vanguard Veteran box, which comes with pretty much 5 sets of every kind of assault weapon (bolt pistol + chainsword, thunderhammer + stormshield, lightning claws plus a few more exotic weapons.

This lets you kit out a regular assault squad of MkIVs, make a MkIV veteran vanguard full of Lightning Claws (because those have the whole arm), and you have leftover weapons for character models to boot.

Also note that the Forge World power weapons packs have arms for two-weapon fighting,, but only 2 sets of bolt pistol and chainsword.