From OMIM
The hereditary spastic paraplegias (SPG, HSP) are a group of clinically and genetically diverse inherited disorders characterized predominantly by progressive lower extremity spasticity and weakness. SPG is classified by mode of inheritance (autosomal dominant, autosomal recessive, and X-linked) and whether the primary symptoms occur in isolation ('uncomplicated') or with other neurologic abnormalities ('complicated'). Pure SPG4 is the most common form of autosomal dominant hereditary SPG, comprising up to 45% of cases (Svenson et al., 2001; Crippa et al., 2006). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).  http://www.omim.org/entry/182601

From MedlinePlus Genetics
Like all hereditary spastic paraplegias, spastic paraplegia type 4 involves spasticity of the leg muscles and muscle weakness. People with this condition can also experience exaggerated reflexes (hyperreflexia), ankle spasms, high-arched feet (pes cavus), and reduced bladder control. Spastic paraplegia type 4 generally affects nerve and muscle function in the lower half of the body only.

Spastic paraplegia type 4 (also known as SPG4) is the most common of a group of genetic disorders known as hereditary spastic paraplegias. These disorders are characterized by progressive muscle stiffness (spasticity) in the legs and difficulty walking. Hereditary spastic paraplegias are divided into two types: pure and complex. The pure types generally involve only spasticity of the lower limbs and walking difficulties. The complex types involve more widespread problems with the nervous system; the structure or functioning of the brain; and the nerves connecting the brain and spinal cord to muscles and sensory cells that detect sensations such as touch, pain, heat, and sound (the peripheral nervous system). In complex forms, there can also be features outside of the nervous system. Spastic paraplegia type 4 is usually a pure hereditary spastic paraplegia, although a few complex cases have been reported.  https://medlineplus.gov/genetics/condition/spastic-paraplegia-type-4