I know about 4 ashram and 4 purusharth but does it somewhere suggest that we should only focus on moksha (as in jainism) and not on purusharth and ashram system.
but does it somewhere suggest that we should only focus on moksha (as in jainism) and not on purusharth and ashram system.
No, it does not.
One can focus wholeheartedly on moksha once he has grown sufficiently old & when he has already fulfilled all his duties first.
When one can fully focus on liberation?
Manu Smriti 6.2. When a householder sees his (skin) wrinkled, and (his hair) white, and. the sons of his sons, then he may resort to the forest.
What happens if someone applies himself fully for liberation without performing his preliminary duties first?
Manu Smriti 6.36. Having studied the Vedas in accordance with the rule, having begat sons according to the sacred law, and having offered sacrifices according to his ability, he may direct his mind to (the attainment of) final liberation
Manu Smriti 6.37. A twice-born man who seeks final liberation, without having studied the Vedas, without having begotten sons, and without having offered sacrifices, sinks downwards
Also, one who seeks moksha for oneself without clearing the three debts (debts to gods, sages and ancestors) first sinks to hell as said in the following verse:
6.35. When he has paid the three debts, let him apply his mind to (the attainment of) final liberation; he who seeks it without having paid (his debts) sinks downwards.
However, in whatever ashrama one lives in Hinduism always emphasizes on collecting spiritual merit because:
4.239. For in the next world neither father, nor mother, nor wife, nor sons, nor relations stay to be his companions; spiritual merit alone remains (with him).
Note- All verses are from Manu Smriti.