you can count the beat 1-2-3-4, but it will be much easier to count the sub-beats as this same pattern is repeated 4 times in one measure)
1-2-3-4-5-6 (4 x 6 sextuplets in one measure, some count the first of the sextuplets numbering the 4/4th notes:
1-2-3-4-5-6, 2-2-3-4-5-6, 3-2-3-4-5-6, *4**-2-3-4-5-6
or you could even count twice to 3 for one sextuplets 1-2-3-1-2-3 twice 3 triplets for a pattern of one beat (4 times in one measure!)
in any case it is the sub-beats that you must have in your mind and that you can feel in your fingers and that you can hear as rhythm pattern!
If you read and play in the way described above, the lower notes will have to be played exactly on that single sextuplet note where it is written:
1st and 3rd example: if you count just 1-2-3-1-2-3 the lower note will always be placed together with 1
in the 2nd example it will come together with 1 and 3 and the second note will be hold along the last sextuplet notes: 1-2-3-4-5-6