With Dhanu Lagna, Buddha, and the Sun in the 4th house, can I wear ruby and emerald together?

Wearing a stone is meant to do the following

Strengthen a weak planet
Invigorate a strong planet even further
Wear a stone to strengthen a specific planet during Mahadasha, Antardasha or Transit.

Now, coming to the question about wearing a ring to benefit from placement

Sun is a malefic and in a 4th, 7th or even 9th House – it impacts the house negatively also. Strengthening it, as the 4th House is Pisces Zodiac, may not be that helpful
For wearing yellow emerald, we need to see the condition of Jupiter and its association with other planets. If there is influence of Rahu on Jupiter through drishti or placement, then Rahu will also start acting with Jupiter, which will nullify the benefits
For wearing green emerald, we need to understand that Mercury in Pisces is its Marna Karaka Sthan(a weak position). Strengthening it may be easier said than done

Having said this, focus on your karma and leave the rest to the Almighty

Hope this helps

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God Bless!