Lord Vishnu as he originally is in his abode is 4 handed. However he adopts different forms which are not all four handed. I will describe a few of his non-for handed forms.
The eight armed form in which he appeared before Daksha Prajapati:
O Mahārāja Parīkṣit, best of the Kuru dynasty, the Lord’s lotus feet rested on the shoulders of His carrier, Garuḍa, and He appeared with eight long, mighty, very beautiful arms. In His hands He held a disc, conchshell, sword, shield, arrow, bow, rope and club — in each hand a different weapon, all brilliantly shining. His garments were yellow and His bodily hue deep bluish.
-Srimad Bhagavatam 6.4.35
The thousand handed form that stood on the summit of the Mandara mountain during the Samudra Manthana:
Manifesting Himself with a thousand hands, the Lord then appeared on the summit of Mandara Mountain, like another great mountain, and held Mandara Mountain with one hand.
- Srimad Bhagavatam 8.7.12
As the son of Kausalya and Dasharatha, Rama has two arms:
After offering the wooden shoes before Lord Rāmacandra, Lord Bharata stood with folded hands, His eyes full of tears, and Lord Rāmacandra bathed Bharata with tears while embracing Him with both arms for a long time.
-Srimad Bhagavatam 9.10.39
As the son of Devaki and Vasudeva, Krishna is requested to hide his Chaturbhuja form and become and ordinary child:
O my Lord, You are the all-pervading Supreme Personality of Godhead, and Your transcendental four-armed form, holding conchshell, disc, club and lotus, is unnatural for this world. Please withdraw this form [and become just like a natural human child so that I may try to hide You somewhere].
-Srimad Bhagavatam 10.3.30
