The SBA now possess multiple vascular loops in which blood is actively circulating, on the ventral side of the usually partially inflated swim bladder ([AMA & derivatives], and poorly visualized in [4 dpf overview]). The right and left SIV, which are the only vessels absorbing yolk after 3.5 or 4 dpf, empty separately into the hepatic sinusoids via the two HPV at the anterior most end of each SIV (discussed above in 3-3.5 dpf section). With the reduction of the yolk, the left SIV shows a degenerative tendency at 4 dpf. The caudal portions of the left SIV become disconnected from one another and form separate segments at 4.5 or 5 dpf ([4 dpf overview], [4.5 dpf overview-left side], [intestinal vasculature]). These disconnected left caudal SIV segments will empty into the right SIV via communicating veins between the right and left SIV along the ventral gut wall (see 5 dpf+ overview diagrams and [intestinal vasculature]). Eventually, the remnants of the left SIV merge completely with the right SIV, and all of the venous return from the caudal gut to the liver is routed solely through the right SIV. Although the cranial most part of the left SIV will continue to drain directly into the liver as the primary HPV, portions of the left SIV on the cranial half of midgut also degenerate, and the cranial and caudal left SIV are disconnected. The midgut itself begins to extend left-ventrad in an arch and rotates clockwise on its longitudinal axis (seen from behind) at 4 dpf. With the reduction of the left anterior SIV, a few intestinal veins newly develop on the ventral wall of midgut to empty their blood into the liver. As mentioned above, this process correlates with the reduction of yolk and the positional change of the midgut (described diagrammatically in [intestinal vasculature]).

The parallel ABA and ABF of the third and forth aortic arches at least are completely separated from one another, and all flow from ABA to ABF becomes routed through ALB capillaries [4 dpf arches, ventral]. The ALB loops are lengthening and extending further caudalwards, and additional ALB loops continue to appear between the ABA and ABF in the four caudal aortic arches [3.5 dpf arch-head], [4 dpf arches]. All of these changes have the effect of increasing the vascular surface area available for gas exchange in this, the future gill circulation.

In the head a pair of new arteries, the nasal arteries (NA) take off from the PICA at 4.0 dpf (see [6.5 dpf head dorsal multilayer composite], also [4.5 dpf head dorsal multilayer composite], although the root of the NA from the PICA is not shown in the latter figure). The NA travel rostrally, then pass along the right and left walls of the nasal sac at the most rostral end of the head. From the nasal sac, the NA flows into the nasal veins (NV). The NV rises dorsally around the front of the head, draining into the ACeV and eventually into the PMBC on either side. The NA and NV are actually the most rostrally projecting set of vessels in the embryo from this stage until at least 7 dpf. A large vascular plexus around the choroid derives from the remnant of the PMsA, which now drains the mesencephalic central arteries, as mentioned above. The DCV also drains into this plexus, which continues to elaborate adjacent to the pigment epithelium of the eye through at least 7 dpf. With the development of the choroid plexus, the ophthalmic vein separates ventrally from the remnant of the PMsA as the robust venous drainage for the plexus ([4.5 dpf head dorsal multilayer composite], lower middle layer, more easily visualized in [6.5 dpf head dorsal multilayer composite], lower middle layer). The PMBC, ophthalmic vein and distal remnant of the PMsA join together further caudally at the PMBC-PHBC junction [4.5 dpf head dorsal multilayer composite], lower 2 layers. The formation of the choroidal vasacular system will be described in more detail in a later publication (S. Isogai, unpublished results).

As described earlier, blood flows in a rostral direction through the rostral PHBC, emptying into the rostral end of the PHS at the base of the MCeV [4.5 dpf head dorsal multilayer composite], lower middle layer. The middle and posterior segments of the PHBC drain via the PCeV [4.5 dpf head dorsal multilayer composite], upper middle layer. Although the portions of the PHBC draining into the PHS or PCeV have not yet completely separated from one another, they will generally do so within the next 2-3 days. The link from the extreme caudal end of the PHBC to the DLAV is no longer present, with the BA having appropriated this link [4.5 dpf head dorsal multilayer composite], upper 2 layers. However, after 5 dpf caudal portions of the PHBC make new connections through to the trunk via vascular segments linking them to the VTA along the ventral wall of the spinal cord [head-trunk connections] and [4.5 dpf head dorsal multilayer composite], upper layer. In the dorsal head, the paired MsV and the two PCeV all join together into a single vessel, the dorsal longitudinal vessel (DLV) for the final portion of their length around the DMJ. Otherwise, although there continues to be an elaboration of smaller caliber vessels throughout the head the overall "wiring pattern" of major head vessels is still largely the same as that seen at 2-2.5 dpf, as noted before.