From the data I have seen you will have less than 40J CVN impact energy @ 23 deg C for 4140 quenched and tempered (actually the impact data of 40J was for a lower strength UTS of 120 Ksi Q&T). At your stated strength levels and impact requirement, you need an alloy steel with Ni added to assure strength and adequate toughness, and good weldability, like AISI Type 4340.
Welding in this high strength condition will present a serious challenges. Off-hand, if you were to upgrade to an AISI Type 4340 alloy steel tube, the top end filler metal that you could use without any form of PWHT is probably an SFA 5.28 ER120S-1 weld wire. The tensile strength will be 120 Ksi minimum, as welded, which will be less than 160 Ksi UTS. You will have to decide if you can accept this material in your design, or modify the component design to reduce service stresses. Other options include fastening versus welding.
If you decide to weld, the welding should be performed using the GTAW process with a 450 deg F local preheat. The GTAW process will provide a better chance of maintaining base metal HAZ impact properties. The ER120S-1 weld metal toughness will be well above 60J at RT. Use stringer beads with about a 50% bead overlap to sequentially deposit the weld. The reason for this technique is to attempt to soften the base metal heat affected zone using heat from well placed stringer beads. Keep the interpass temperature below 600 deg F. After the completion of welding, slow cool and perform a wet fluorescent MT.
