You'd need three 2-way splitters. As a 4-way may very well be three 2-way splitters internally, I doubt there's much difference.
If I were determined to join 4 antennas, I'd probably want to join/balance them in pairs, then join/balance the two pairs together; get each pair working well, then get the two pairs working together. Three 2-way splitters would give you that flexibility.
Note: I'd have to be determined. I don't know that I'd even attempt it without some pretty expensive testing and measuring equipment.