You choose the first four (4) placegetters past the post in the order you think they will finish. If your four selections finish in the correct order, you win the dividend.
With a box First4 you can select as many runners as you like. Providing that four of your selections finish first, second, third and fourth you win the dividend.
With a Multiple First4, any number of selections can be taken for first, second, third and fourth. Providing the selections that you nominate for each placing actually finish in that placing, then you will win the dividend. For example, you might select two (2) runners for first, four (4) runners to fill second placing, six (6) runners to fill third and eight (8) runners to finish fourth.
If you think there's a 'sure thing' to win the First4 race, then a Banker First4 is ideal for you. The 'Banker' would be the 'sure thing' and you could couple it with three (3) or more other runners to fill the second, third and fourth placing. Your 'Banker' must win the race and your other selections must finish second, third and fourth for you to win the dividend.
If you have a 'sure thing' to finish in the first four placings you can take it as a Roving Banker First4 with three or more other runners. Providing your Roving Banker selection runs first, second, third or fourth and your 'other' selections fill the other placings, you win the First4. Likewise, if you have two (2) 'sure things', you can take those two as a Double Roving First4 with two or more other runners. Providing your two (2) Double Roving selections run two of the first four placings and your other selections fill the remaining two placings, you win the First4.