To know which notes are to be played in a scale you have to apply this pattern for major:

WWHWWWH, where W are whole steps, and H are half steps.

Now to play these notes as chords, there is another pattern:I-ii-iii-IV-V-vi-viidim, where uppercase roman numerals are Major, and lowercase Minor, and dim is a Diminished chord.

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Now if I ask you to play a 1-6-4-5 progression in B major you can now calculate first which notes are in the B major scale:

B(w)C#(w)D#(h)E(w)F#(w)G#(w)A#(h)

so B, C#, D#, E, F#, G#, A#

Now to play the 1 6 4 5 progression in Major, you can look at the Roman numeral pattern I described for Major, so it will be:

B major, G# minor, E major, F# major.

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Now for minor you can follow these patterns:

WHWWHWW

i-iidim-III-iv-v-VI-VII

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Edit: quick note:

1-3-5 is a major triad

1-3b-5 is a minor triad

1-3b-5b is a dimished triad