The most important thing to remember about ionic compounds is that they must be electrically neutral, i.e. the overall charge of a formula unit must be zero.
Ionic compounds are formed when cations, which are positively charged ions, form ionic bonds with anions, which are negatively charged ions.
The cations and anions are held together by the very powerful electrostatic force of attraction that exists between the opposite charges.
In your case, you know that the cation has a
In order for an ionic compound to form, you need the overall negative charge coming from the anions to balance out the overall positive charge coming from the cation.
In order to achieve that balance, you need to have four anions for every one cation, since
#4 xx (1-) = 4-#
will balance out the
#"Mn"^color(red)(4+)"Br"^color(blue)(1-) implies "Mn"_color(blue)(1)"Br"_color(red)(4) implies "MnBr"_4#
As you can see, one way to remember this is that the charge of the cation becomes the subscript of the anion and vice versa - this is known as the crisscross method.
The name of the compound will be manganese(IV) bromide. The Roman numeral is used here because manganese can have multiple oxidation states.