Support for a move toward $400:

1. Strong fundamentals: UnitedHealth has diversified business (healthcare plans + services, via Optum etc.), strong cash flow, and decent earnings growth. Those help build a base for higher valuations. 

2. Recent upgrades and raised targets: Some upgrades (like at Truist) show increasing confidence. These tend to help with investor sentiment. 

3. Room for multiple expansion: If UNH can demonstrate margin improvement, reduce regulatory risk, or show accelerating growth, the market may reward it with a higher P/E, pushing the price up.

Challenges / what needs to go right / risk factors:

• Regulatory risk is nontrivial for big health insurers. Changes in Medicare/Medicaid policy, reimbursement rules, or government investigations could hurt valuation. 

• The expected growth and earnings need to justify a higher multiple. Any earnings misses, cost inflation (medical expense trend, labor, etc.), or margin pressure could drag performance.

• General market sentiment (interest rates, economic slowdown) could make investors more risk-averse, punish high valuations.

• Some analysts’ targets are already close to current price, which limits upside unless there is strong catalyst.