From the Guidelines

Yes, a patient with a resolved grade 4 splenic laceration can be started on prophylactic anticoagulation after 8 days.

The evidence suggests that spleen trauma without ongoing bleeding is not an absolute contraindication to LMWH-based prophylactic anticoagulation 1.
LMWH-based prophylactic anticoagulation should be started as soon as possible from trauma and may be safe in selected patients with blunt splenic injury undergone to non-operative management (NOM) 1.
The risk of venous thromboembolism (VTE) is high in trauma patients, and prophylactic anticoagulation can help reduce this risk 1.
While there are no standards for the initiation of prophylactic anticoagulation in trauma patients with blunt spleen injuries, the use of heparin and LMWH is common, especially in high-grade lesions 1.
Mechanical prophylaxis is safe and should be considered in all patients without absolute contraindication to its use, and can be combined with chemical prophylaxis 1.
The timing of initiation of prophylactic anticoagulation is not strictly defined, but studies have shown no differences between VTE prophylaxis administered within and after a certain time frame 1.

Mechanical prophylaxis should be considered in all patients without absolute contraindication to its use.
LMWH-based prophylactic anticoagulation can be started as soon as possible from trauma in selected patients with blunt splenic injury.
Risk-benefit balance of reversal of oral anticoagulants should be individualized.
Trauma patients are at high risk of VTE, and prophylactic anticoagulation can help reduce this risk.

From the Research

A patient with a resolved grade 4 splenic laceration can be considered for prophylactic anticoagulation after 8 days, based on the timing of administration of anticoagulation in patients with blunt spleen or liver injuries 2.
The safety of low molecular-weight heparin after blunt liver and spleen injuries has been studied, and early administration may be safe and reduce the incidence of thrombotic complications in patients with blunt spleen and liver injuries 2.
The World Society of Emergency Surgery suggests that DVT and VTE prophylaxis with LMWH can be started within 48-72 hours from hospital admission in patients with high-grade splenic injuries 3.

Early initiation of VTE prophylaxis (<48 hours) is safe in patients with blunt splenic injuries treated nonoperatively 4.
The timing of administration of anticoagulation is not clearly defined when patients have blunt spleen or liver injuries, but early administration may be safe and reduce the incidence of thrombotic complications 2.
Prophylactic anticoagulation can be started within 48-72 hours from hospital admission in patients with high-grade splenic injuries 3.

A retrospective review of trauma patients with blunt splenic injury found that early initiation of VTE prophylaxis does not increase failure of nonoperative management or transfusion requirements in these patients 4.
A systematic review with meta-analysis found that prophylactic angioembolization significantly reduced risk of management failure and decreased patient mortality compared with clinical observation alone in patients with high-grade blunt trauma splenic injuries 5.
A study on the safety of low molecular-weight heparin after blunt liver and spleen injuries found that early administration may be safe and reduce the incidence of thrombotic complications in patients with blunt spleen and liver injuries 2.