Would I get promoted to upper league with the new MMR system by this playing frequency?

I was in master 4v4 (Random) a while ago. I did not play for 1 season and when I get back I got placed in bronze league. Games were too easy for me now but the thing is: I play 1 day a week (~10 games) and I win most of them. I only got to silver first day. 2 weeks later gold. And now I have played more than 10 games in gold and I won most of them but still I'm in gold. I know as the time passes my MMR decays but will I be able to get into higher leagues by playing in this frequency? once a week?

According to Blizzard Forums:

By not playing for an entire season, you wiped your MMR clean.

If you are inactive for an entire season (that is, you stop playing in Season n, you play no games--ranked or unranked--during Season n+1, you come back for Season n+2), your MMR is wiped clean.

Therefore it will take sometime to get to upper leagues. Just keep playing, you eventually will end up where you belong according to your MMR.

For answering your question:

How Frequently Must I Keep Playing to Prevent Decay?

Decay begins to take effect between 2 weeks and 4 weeks of uninterrupted inactivity. We believe the effect to be linear, from a decay of 0 at 2 weeks to about 310 at 4 weeks. The below image illustrates: The red line is the decay rate. x is time in days and y is decay amount. The decay hard caps at the blue and green lines.

So, you're safe playing one day a week. Your MMR won't decay.