Lateral restraint would be ok provided the sheet steel is ok for the compression force generated due to the lateral movement. You would need to check compression buckling in the sheet and make sure you have restraint back to a system that transfers lateral loads suitably to the ground.
I feel without proper stiffeners 4mm sheet( even if continuously welded to the beam) cannot be taken as a reliable lateral constraint.I would prefer not to consider it .
Excerpt from Lui's Handbook of Structural Engineering, "Torsional buckling occurs in members with doubly symmetric sections such as cruciform or built-up shapes with very thin walls. Flexural–torsional buckling occurs in members with singly symmetric cross-sections (e.g., channel,tee, equal-legged angle, double-angle sections) when such sections are buckled about the axis of symmetry and in members with unsymmetric cross-sections (e.g.,unequal-legged L). Normally,torsional buckling of symmetric shapes is not particularly important in the design of hot-rolled compression members. It either does not govern or its buckling strength does not differ significantly from the corresponding weak axis flexural buckling strengths. However, torsional buckling may become important for open sections with relatively thin component plates."
What kind of structural shape are you talking about here?
"Sheesh. Some days it just doesn't pay to get out of bed."
Assume that the thin, 4mm, sheet is on both sides of the compression flange. The side in tension does not need stiffeners. This tension side is sufficient for bracing purposes against twisting and lateral movement.
